more syntax features: keycodes, special, parameters

[clason]

In testing this parser for nvim-treesitter (nvim-treesitter/nvim-treesitter#2679), I noticed that some features are not yet supported, especially

• Parameters as in {pattern}
• Keycodes and chords as in <c-o> and CTRL-O
• Command modifiers as in [range] (that's just a bunch of literals, and so could simply be listed as anonymous nodes in the highlight query)
• optional arguments [optional] argument #109
• modelines? Update repository and add modeline support #125

(See https://github.com/nanotee/vimdoc-notes for the most comprehensive specification of vimdoc I've seen so far.)

[vigoux]

I will use this issue as tracking for missing features in the language if that's okay for you, feel free to add stuff to the original comment.