feat(compiler): Add warning for non-integer array indices

[spotandjake]

This pr adds a basic warning for arr[1.5] and arr[1.5] = 1 along with rational use cases. The purpose of this is to better catch instances where a user is indexing with floats ahead of time and prevent more runtime bugs. You can still achieve the runtime Index not an integer error by doing something like let i = 1.5; arr[i] though. The biggest place thing this helps to prevent is arr[1.0] where a user might assume we cast that to an int like number.

[ospencer]

Because our number system does allow users to end up in situations where they might try to index by a non-integer, I don't think this should be a well-formedness error, but rather a warning. Making arr[1.0] fail to compile introduces some confusion about what the compiler is going to guarantee you.

[spotandjake]

Because our number system does allow users to end up in situations where they might try to index by a non-integer, I don't think this should be a well-formedness error, but rather a warning. Making arr[1.0] fail to compile introduces some confusion about what the compiler is going to guarantee you.

do we currently support raising warnings in normal well_formdness? I can't see this functionality anywhere or would this need to be implemented?

[ospencer]

It's supported, we just don't do it. You can see how to do it in typed_well_formedness.re: https://github.com/grain-lang/grain/blob/main/compiler/src/typed/typed_well_formedness.re#L270-L278

[spotandjake]

Made that change.