Add parsing support for whitespace in macro_rules

[erickt]

This fixes macro_rules with whitespace between the symbols, like:

macro_rules ! macro_with_whitespace {
    ($ x : expr, $ y : expr) => { $ x + $ y }
}

I couldn't find an upstream test that specifically tested for this, but this syntax is accepted by the compiler.

[wetneb]

Hi @erickt,

Thanks a lot for this! It looks good to me. Given that contributions to this repository are infrequently reviewed, I have been maintaining a fork of this grammar where I have integrated your changes (some of them were already done, so the diff is smaller).

The changes are released on crates.io, PyPI and Codeberg. If you need them to be published elsewhere, let me know.