Complete support for Starlark type annotations (#1488)

This change introduces parsing, AST, formatting, and linting support for
experimental Bazel Starlark type annotations
(https://github.com/bazelbuild/bazel/issues/27370) and Buck2 type
syntax:

- Type alias statements (`type OptionalDict[T, U] = dict[T, U] | None`).
- Generic `def` statements (`def foo[T, U](x: T) -> U`).
- Var statements (`x: list[int]`)
- `type_*` AST nodes to cleanly distinguish type syntax from standard
expressions. In particular, this allows us to improve formatting for
complex type applications, and to allow the ellipsis (`...`) only in
type expressions.

Not supported yet: `cast` and `isinstance` expressions - these will be
added in a follow-up PR; supporting them is a bit tricky due to the
limitations of yacc's LALR(1) grammar.

Disclosure: Gemini-assisted.

TAG=agy
CONV=ae2ff586-d57d-4fd4-a22a-883aec5d0802

## Buildtools PR checklist

- [x] The code in this PR is covered by unit/integration tests.
- [x] I have tested these changes and provide testing instructions
below.
- [ ] I have either responded to, or resolved all Gemini comments on the
PR.
- [ ] I have read Google Eng Practices on [Small
Changes](https://google.github.io/eng-practices/review/developer/small-cls.html),
this PR either follows these guidelines or the description provides
reasoning for why they can not be followed.

@brandjon @susinmotion FYI