- Don't assume b:undo_ftplugin is set when first modifying it.
- Don't assume b:undo_ftplugin already contains some resetting.

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 '|'.

This variable was only meant for easy testing during the development
cycle for treesitter highlighting while Lua was the only parser useable
for daily driving. Now that we have a good vimdoc parser, this approach
simply doesn't scale and should be removed sooner rather than later.

Instead of setting this variable, people for now should add the autocommand
directly to their config:
```lua
vim.api.nvim_create_autocmd('FileType', {
  pattern = 'lua', -- or { 'lua', 'help' }
  callback = function() vim.treesitter.start() end,
})
```
(or put `vim.treesitter.start()` in an `ftplugin`).

* Add vim.treesitter.start() for starting treesitter highlighting via
  ftplugin or autocommand (can be extended later for fold, indent,
  matchpairs, ...)
* Add vim.treesitter.stop() for manually stopping treesitter
  highlighting
* Enable treesitter highlighting for Lua if
  `vim.g.ts_highlight_lua = true` is set in `init.lua`