This allows whitespace between the brackets and the register.
This also fixes#531
Note that `$ff00 + c` is still treated as a single token, because trying to
use an expression on the left side causes a shift/reduce conflict.
This isn't great, but most people seem to be either used to it as-is, or
using the new `ldh a, [c]` syntax.
If this causes problems with a lexer rewrite, it'll be deprecated; but for
now, keep it around, as the support is clunky but bearable.
