keyword `in` or `∈` for membership checking

Originally from [here](https://github.com/Macaulay2/M2/pull/3584):

> I think `∈` should be a synonym for `in` so you can write `for x ∈ L do ...`, though maybe we can also add new syntactic sugar `x in L` which calls `isMember`!

To clarify, there would be two situations for `in`:
- inside a for clause, acts as "for every element in L"
- outside a for clause, acts as "whether element is a member in L"

No source file, test, or entry point is identified in the issue. Start by locating the parser and existing for-clause handling, then determine how membership is represented; done means `∈` works in for clauses and `in` supports membership checks outside them.

Written by the indexing model from the issue text.