refactor(#2886): multi instance: node group functions refactoring (#2959)

* move last_group_node to DirectoryNode

* move add BaseNode:as and more doc

* revert parameter name changes

* revert parameter name changes

* add Class

* move group methods into DN

* tidy group methods

* tidy group methods

* tidy group methods

* tidy group methods

* parent is DirectoryNode

* tidy expand all

* BaseNode -> Node

* move watcher to DirectoryNode

* last_group_node is DirectoryNode only

* simplify create-file

* simplify parent

* simplify collapse-all

* simplify live-filter

* style

lua/nvim-tree/class.lua

@@ -0,0 +1,40 @@
+---Generic class, useful for inheritence.
+---@class (exact) Class
+---@field private __index? table
+local Class = {}
+
+---@param o Class?
+---@return Class
+function Class:new(o)
+  o = o or {}
+
+  setmetatable(o, self)
+  self.__index = self
+
+  return o
+end
+
+---Object is an instance of class
+---This will start with the lowest class and loop over all the superclasses.
+---@param class table
+---@return boolean
+function Class:is(class)
+  local mt = getmetatable(self)
+  while mt do
+    if mt == class then
+      return true
+    end
+    mt = getmetatable(mt)
+  end
+  return false
+end
+
+---Return object if it is an instance of class, otherwise nil
+---@generic T
+---@param class T
+---@return `T`|nil
+function Class:as(class)
+  return self:is(class) and self or nil
+end
+
+return Class