Interop Gen: Support Mapped Types

[nikeokoronkwo]

This issue is for adding support for mapped types in TS.

Depending on the key representation of this given type, we could either have it as:

• A JSRecord (not to be confused with the current implementation of a JS record in package:js_interop as this one will need generics on both key and value)
• An anonymous object representation for cases of keyof <type>

This also considers modifications on readonly, nullability, typing (using as casts), and others.

If it is possible to get some resolution of the type by the type checker, it would be helpful to do so. Keys that resolve to never should be removed.

[harrshita123]

@nikeokoronkwo
I am interested in working on this issue.

Before starting implementation, I would like to clarify:

• Preferred Dart representation for mapped types (JSRecord vs anonymous object).
• Whether we should initially support only resolvable keyof cases.
• How strict the first version should be about readonly / nullability.

Happy to start with a minimal implementation if that is preferred.