fix(runtime): update b:undo_ftplugin in Lua runtime files (#29529)

Related to #29506, but adding vim.treesitter.stop() to b:undo_ftplugin
doesn't solve the problem yet.

runtime/ftplugin/help.lua

@@ -30,3 +30,5 @@ end
 vim.keymap.set('n', 'gO', function()
   require('vim.vimhelp').show_toc()
 end, { buffer = 0, silent = true })
+
+vim.b.undo_ftplugin = vim.b.undo_ftplugin .. ' | nunmap <buffer> gO'