Tweaks for the new filetype handling.

1. The ftplugin does not have to source ftplugins for HTML. This is
handled by setting ft=html.twig when editing foo.html.twig and bar.twig,
and it is wrong when editing foo.js.twig.
2. Do not rely on ftplugins already defining b:undo_ftplugin.
3. OMG, did I leave off "let" in the syntax file?!?