feat(#2948): add custom decorators, :help nvim-tree-decorators (#2996)

* feat(#2948): add UserDecorator, proof of concept

* feat(#2948): add UserDecorator, proof of concept

* feat(#2948): add UserDecorator, proof of concept

* feat(#2948): add UserDecorator

* feat(#2948): add UserDecorator

* feat(#2948): add UserDecorator

* feat(#2948): add Decorator node icon override

* feat(#2948): add nvim_tree.api.* node classes

* feat(#2948): extract _meta following nvim pattern

* feat(#2948): extract _meta following nvim pattern

* feat(#2948): add decorator registry and order

* feat(#2948): add decorator registry and order

* feat(#2948): tidy

* feat(#2948): document API

* feat(#2948): document API

* feat(#2948): document API

* feat(#2948): pass api nodes to user decorators

* feat(#2948): document API

* feat(#2948): use renderer.decorators to define order and register

* feat(#2948): tidy decorator args and complete documentation

* feat(#2948): decorator classes specified by prefix rather than suffix

* feat(#2948): improve doc

* feat(#2948): improve doc

* feat(#2948): improve doc

* feat(#2948): additional user decorator safety

* feat(#2948): create nvim_tree.api.decorator.UserDecorator class in API, add :extend

* feat(#2948): improve doc

lua/nvim-tree/renderer/decorator/init.lua

@@ -1,48 +1,52 @@
 local Class = require("nvim-tree.classic")
 
----@alias DecoratorRange "none" | "icon" | "name" | "all"
----@alias DecoratorIconPlacement "none" | "before" | "after" | "signcolumn" | "right_align"
-
 ---Abstract Decorator
----Uses the factory pattern to instantiate child instances.
 ---@class (exact) Decorator: Class
----@field protected explorer Explorer
 ---@field protected enabled boolean
----@field protected range DecoratorRange
----@field protected icon_placement DecoratorIconPlacement
+---@field protected highlight_range nvim_tree.api.decorator.HighlightRange
+---@field protected icon_placement nvim_tree.api.decorator.IconPlacement
 local Decorator = Class:extend()
 
 ---@class (exact) DecoratorArgs
 ---@field explorer Explorer
 
----@class (exact) AbstractDecoratorArgs: DecoratorArgs
----@field enabled boolean
----@field hl_pos DecoratorRange
----@field icon_placement DecoratorIconPlacement
-
----@protected
----@param args AbstractDecoratorArgs
-function Decorator:new(args)
-  self.explorer       = args.explorer
-  self.enabled        = args.enabled
-  self.range          = args.hl_pos
-  self.icon_placement = args.icon_placement
+---Abstract icon override, optionally implemented
+---@param node Node
+---@return HighlightedString? icon_node
+function Decorator:icon_node(node)
+  return self:nop(node)
 end
 
----Maybe highlight groups
+---Abstract icons, optionally implemented
+---@protected
 ---@param node Node
----@return string|nil icon highlight group
----@return string|nil name highlight group
-function Decorator:groups_icon_name(node)
+---@return HighlightedString[]? icons
+function Decorator:icons(node)
+  self:nop(node)
+end
+
+---Abstract highlight group, optionally implemented
+---@protected
+---@param node Node
+---@return string? highlight_group
+function Decorator:highlight_group(node)
+  self:nop(node)
+end
+
+---Maybe highlight groups for icon and name
+---@param node Node
+---@return string? icon highlight group
+---@return string? name highlight group
+function Decorator:highlight_group_icon_name(node)
   local icon_hl, name_hl
 
-  if self.enabled and self.range ~= "none" then
-    local hl = self:calculate_highlight(node)
+  if self.enabled and self.highlight_range ~= "none" then
+    local hl = self:highlight_group(node)
 
-    if self.range == "all" or self.range == "icon" then
+    if self.highlight_range == "all" or self.highlight_range == "icon" then
       icon_hl = hl
     end
-    if self.range == "all" or self.range == "name" then
+    if self.highlight_range == "all" or self.highlight_range == "name" then
       name_hl = hl
     end
   end
@@ -52,13 +56,13 @@ end
 
 ---Maybe icon sign
 ---@param node Node
----@return string|nil name
+---@return string? name
 function Decorator:sign_name(node)
   if not self.enabled or self.icon_placement ~= "signcolumn" then
     return
   end
 
-  local icons = self:calculate_icons(node)
+  local icons = self:icons(node)
   if icons and #icons > 0 then
     return icons[1].hl[1]
   end
@@ -66,56 +70,40 @@ end
 
 ---Icons when "before"
 ---@param node Node
----@return HighlightedString[]|nil icons
+---@return HighlightedString[]? icons
 function Decorator:icons_before(node)
   if not self.enabled or self.icon_placement ~= "before" then
     return
   end
 
-  return self:calculate_icons(node)
+  return self:icons(node)
 end
 
 ---Icons when "after"
 ---@param node Node
----@return HighlightedString[]|nil icons
+---@return HighlightedString[]? icons
 function Decorator:icons_after(node)
   if not self.enabled or self.icon_placement ~= "after" then
     return
   end
 
-  return self:calculate_icons(node)
+  return self:icons(node)
 end
 
 ---Icons when "right_align"
 ---@param node Node
----@return HighlightedString[]|nil icons
+---@return HighlightedString[]? icons
 function Decorator:icons_right_align(node)
   if not self.enabled or self.icon_placement ~= "right_align" then
     return
   end
 
-  return self:calculate_icons(node)
-end
-
----Maybe icons, optionally implemented
----@protected
----@param _ Node
----@return HighlightedString[]|nil icons
-function Decorator:calculate_icons(_)
-  return nil
-end
-
----Maybe highlight group, optionally implemented
----@protected
----@param _ Node
----@return string|nil group
-function Decorator:calculate_highlight(_)
-  return nil
+  return self:icons(node)
 end
 
 ---Define a sign
 ---@protected
----@param icon HighlightedString|nil
+---@param icon HighlightedString?
 function Decorator:define_sign(icon)
   if icon and #icon.hl > 0 then
     local name = icon.hl[1]