Make parser reusable

[danielshahaf]

The main highlighter incorporates a zsh syntax parser. Separate that parser into an independently reusable component. The lexer is mostly handled by bufferwords() aka ${(z)}, although subshells and command substitutions are not currently recursed into (they're returned as a single token).

I do not have a concrete use-case for that at the moment, but I suspect one of these days somebody on zsh-workers@ will have an idea that will be much easier to implement were this done.

Note: it would be nice to reuse Src/parse.c's code, or to share code with compsys (get_comp_string).

[danielshahaf]

Parsing parameter expansions should probably be its own project. :-)