feat: add sort_by "filetype" (#2302)

* feat: adds new type of sorting based on vim's filetype detection

* fix(ft/sorter): fallbacks to C.name when both ft's are nil or equal

* feat: adds new type of sorting based on vim's filetype detection

---------

Co-authored-by: Alexander Courtis <alex@courtis.org>

doc/nvim-tree-lua.txt

@@ -568,8 +568,8 @@ Reloads the explorer every time a buffer is written to.
 
 *nvim-tree.sort_by*
 Changes how files within the same directory are sorted.
-Can be one of `name`, `case_sensitive`, `modification_time`, `extension` or a
-function.
+Can be one of `name`, `case_sensitive`, `modification_time`, `extension`,
+`filetype` or a function.
   Type: `string` | `function(nodes)`, Default: `"name"`
 
   Function may perform a sort or return a string with one of the above
@@ -577,6 +577,7 @@ function.
   - `absolute_path`: `string`
   - `executable`:    `boolean`
   - `extension`:     `string`
+  - `filetype`:      `string`
   - `link_to`:       `string`
   - `name`:          `string`
   - `type`:          `"directory"` | `"file"` | `"link"`