vim: Handle box expressions specially

vim: Handle box expressions specially

Attempt to highlight the placement expression in a `box (expr) foo`
expression. Also treat GC as a keyword within the placement expression.

This doesn't work correctly for arbitrary expressions. Notably, this
makes no attempt at balancing delimiters. I believe handling that will
require rewriting the syntax rules to add a region for every pair of
delimiters. That may be a desirable thing to do in the end, because we
can then rewrite our indent rules based on the syntax and get rid of
cindent(), but for the time being, we'll just live with the limitation.