fix(runtime): stop treesitter highlight in b:undo_ftplugin #38459

Co-authored-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>

runtime/ftplugin/markdown.lua

@@ -12,5 +12,6 @@ vim.keymap.set('n', '[[', function()
 end, { buf = 0, silent = false, desc = 'Jump to previous section' })
 
 vim.b.undo_ftplugin = (vim.b.undo_ftplugin or '')
+  .. '\n call v:lua.vim.treesitter.stop()'
   .. '\n sil! exe "nunmap <buffer> gO"'
   .. '\n sil! exe "nunmap <buffer> ]]" | sil! exe "nunmap <buffer> [["'