feat: add fern extension (#398)

* Add fern extension

Fern[^1] is general purpose tree view that can browse and manipulate
file trees. This extension provides a simple and useful statusline for
fern buffers.

[^1]: https://github.com/lambdalisue/fern.vim

* Reuse nerdtree code for fern

* doc: mention fern in README

Co-authored-by: shadmansaleh <13149513+shadmansaleh@users.noreply.github.com>

README.md

@@ -605,6 +605,7 @@ extensions = {'quickfix'}
 <summary><b>Available extensions</b></summary>
 
 * chadtree
+* fern
 * fugitive
 * fzf
 * nerdtree

doc/lualine.txt

@@ -604,6 +604,7 @@ Available extensions ~
 
 
 - chadtree
+- fern
 - fugitive
 - fzf
 - nerdtree

lua/lualine/extensions/fern.lua

@@ -0,0 +1,11 @@
+-- MIT license, see LICENSE for more details.
+-- Extension for fern file explorer.
+local nerdtree = require 'lualine.extensions.nerdtree'
+
+local M = {}
+
+M.sections = vim.deepcopy(nerdtree.sections)
+
+M.filetypes = { 'fern' }
+
+return M