Improve parsing of list shapes

[danog]

Fixes parsing of list shape phpdocs with optional keys: list{0: int, 1?: int, 2?: string}

[weirdan]

Why would we need this?

[danog]

This is actually a valid usecase that's already documented and works properly in assertions, but not in docblocks.

[weirdan]

Would you mind adding a test for list{2: string, 1: string, 0: string}? I'm not even sure whether that's valid (if order is not preserved) or invalid (if it is) list. That's the case that would have been prevented if we did not support keys in list{}.

[danog]

Done!

[orklah]

Thanks!