feat: add IndexExpr support to constant_fold_expr [1/2]

[BobTheBuidler]

This PR adds support for IndexExpr in constant_fold_expr and support for constant folding in transform_index_expr

[JukkaL]

Please add some test cases -- the logic is non-trivial so it would be good to have test coverage.

[JukkaL]

I think it would be better to check if base is a Sequence -- you can index an int, for example.

[BobTheBuidler]

If we can merge #19930 first I would be able to re-use the same run test, I think that's probably best for the sake of deduplication. Wdyt?

[hauntsaninja]

Jukka's comment about checking base is a sequence still applies, right?

[BobTheBuidler]

Sequence / dict, yeah. It isn't just Sequence because #19864 enables quasi-folding over dicts

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[BobTheBuidler]

@hauntsaninja did I perhaps misunderstand what you were asking about? Anything still blocking this one?

[hauntsaninja]

Yeah, I understood Jukka's suggestion to be to use if isinstance(base, (Sequence, Mapping)) etc to avoid the type ignore comments on L102 and L109 #19982 (comment)