[ty] New Type variant for TypedDict (#19733)

## Summary

This PR adds a new `Type::TypedDict` variant. Before this PR, we treated
`TypedDict`-based types as dynamic Todo-types, and I originally planned
to make this change a no-op. And we do in fact still treat that new
variant similar to a dynamic type when it comes to type properties such
as assignability and subtyping. But then I somehow tricked myself into
implementing some of the things correctly, so here we are. The two main
behavioral changes are: (1) we now also detect generic `TypedDict`s,
which removes a few false positives in the ecosystem, and (2) we now
support *attribute* access (not key-based indexing!) on these types,
i.e. we infer proper types for something like
`MyTypedDict.__required_keys__`. Nothing exciting yet, but gets the
infrastructure into place.

Note that with this PR, the type of (the type) `MyTypedDict` itself is
still represented as a `Type::ClassLiteral` or `Type::GenericAlias` (in
case `MyTypedDict` is generic). Only inhabitants of `MyTypedDict`
(instances of `dict` at runtime) are represented by `Type::TypedDict`.
We may want to revisit this decision in the future, if this turns out to
be too error-prone. Right now, we need to use `.is_typed_dict(db)` in
all the right places to distinguish between actual (generic) classes and
`TypedDict`s. But so far, it seemed unnecessary to add additional `Type`
variants for these as well.

part of https://github.com/astral-sh/ty/issues/154

## Ecosystem impact

The new diagnostics on `cloud-init` look like true positives to me.

## Test Plan

Updated and new Markdown tests

crates/ty_python_semantic/src/semantic_model.rs

@@ -223,7 +223,8 @@ impl<'db> Completion<'db> {
                 Type::NominalInstance(_)
                 | Type::PropertyInstance(_)
                 | Type::Tuple(_)
-                | Type::BoundSuper(_) => CompletionKind::Struct,
+                | Type::BoundSuper(_)
+                | Type::TypedDict(_) => CompletionKind::Struct,
                 Type::IntLiteral(_)
                 | Type::BooleanLiteral(_)
                 | Type::TypeIs(_)