feat: add NvimTreeSymlinkIcon (#2198)

* #2193 Add the ability to change the icon color for "symlink_formatted"

* Remove defaults and update doc #2198

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Co-authored-by: DoctorKnowsBetter <you@example.com>
Co-authored-by: Alexander Courtis <alex@courtis.org>

lua/nvim-tree/renderer/builder.lua

@@ -173,8 +173,9 @@ function Builder:_build_symlink(node)
   end
 
   local link_highlight = "NvimTreeSymlink"
+  local icon_hl = "NvimTreeSymlinkIcon"
 
-  return { str = icon }, { str = symlink_formatted, hl = link_highlight }
+  return { str = icon, hl = icon_hl }, { str = symlink_formatted, hl = link_highlight }
 end
 
 ---@param node table