Pack indexing

[seishun]

Fixes #677.

I'm not happy with the -> being mistagged, but I'm also not excited about fixing it for pack-index-specifier while keeping it broken for decltype-specifier (see computed-type-specifier, simple-type-specifier, type-specifier, type-specifier-seq, type-id, trailing-return-type), and I'm not sure if trying to update the pattern in qualified_type is the right approach.

Also I'll need help with scope naming because I don't understand the naming scheme.

[jeff-hykin]

I'm not sure if trying to update the pattern in qualified_type is the right approach

It is, but this PR is a good starting point cause its a lot easier to understand.

And yeah dont worry about the scope naming, theyre all just made up names with some very rough/vague patterns.

[seishun]

How about meta.pack.indexing for the whole expression, variable.other.pack for the pack (even though it's not a variable in any meaningful sense...) and punctuation.definition.begin.bracket.square.pack.indexing and punctuation.definition.end.bracket.square.pack.indexing for the opening and closing bracket respectively?