[ty] Default-specialization of generic type aliases (#21765)
## Summary
Implement default-specialization of generic type aliases (implicit or
PEP-613) if they are used in a type expression without an explicit
specialization.
closes https://github.com/astral-sh/ty/issues/1690
## Typing conformance
```diff
-generics_defaults_specialization.py:26:5: error[type-assertion-failure] Type `SomethingWithNoDefaults[int, str]` does not match asserted type `SomethingWithNoDefaults[int, DefaultStrT]`
```
That's exactly what we want :heavy_check_mark:
All other tests in this file pass as well, with the exception of this
assertion, which is just wrong (at least according to our
interpretation, `type[Bar] != <class 'Bar'>`). I checked that we do
correctly default-specialize the type parameter which is not displayed
in the diagnostic that we raise.
```py
class Bar(SubclassMe[int, DefaultStrT]): ...
assert_type(Bar, type[Bar[str]]) # ty: Type `type[Bar[str]]` does not match asserted type `<class 'Bar'>`
```
## Ecosystem impact
Looks like I should have included this last week :sunglasses:
## Test Plan
Updated pre-existing tests and add a few new ones.
