feat(renderer): add renderer.indent_width (#1505)

* feat: add config option for a tree indent width

add 'indent_width' option to configure visible indent for tree nesting
levels (default is 2).

* add 'bottom' char for a corner extension

* apply stylua formatting

* provide value constraints in documentation

* limit minimal indent width

* make marker symbols have one utf8 char width

* match stylua formatting

* add the commentary regarding utf-8 first symbol match

lua/nvim-tree/renderer/builder.lua

@@ -258,9 +258,9 @@ function Builder:_build_line(node, idx, num_children)
   self.index = self.index + 1
 
   if node.open then
-    self.depth = self.depth + 2
+    self.depth = self.depth + 1
     self:build(node)
-    self.depth = self.depth - 2
+    self.depth = self.depth - 1
   end
 end