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/lua.lua

@@ -1,2 +1,4 @@
 -- use treesitter over syntax
 vim.treesitter.start()
+
+vim.b.undo_ftplugin = vim.b.undo_ftplugin .. ' | call v:lua.vim.treesitter.stop()'