Starlark type checker should support something like `reveal_type()`

Having something like mypy or pyrefly's `reveal_type()` would both aid users and massively simplify some of Bazel's type system tests.

Sub-issue of #27370

@brandjon FYI

Start by reading the Starlark type checker context and the parent issue #27370, since no implementation files or tests are identified here. Define the expected reveal_type()-like behavior and its test coverage; done means the behavior is specified and the relevant type-system tests can exercise it.

Written by the indexing model from the issue text.