fix(runtime): stop treesitter highlight in b:undo_ftplugin (#29533)

It seems that nvim-treesitter stops treesitter highlight when changing
filetype, so it makes sense for builtin ftplugins to do this as well.

Use :call and v:lua here to allow separation with '|'.

runtime/ftplugin/help.lua

@@ -31,4 +31,5 @@ vim.keymap.set('n', 'gO', function()
   require('vim.vimhelp').show_toc()
 end, { buffer = 0, silent = true })
 
+vim.b.undo_ftplugin = vim.b.undo_ftplugin .. ' | call v:lua.vim.treesitter.stop()'
 vim.b.undo_ftplugin = vim.b.undo_ftplugin .. ' | nunmap <buffer> gO'