[ty] Include constraint-set assignability in more checks (#28642)
## Summary
This PR is inspired by https://github.com/astral-sh/ruff/pull/28638 by
@ibraheemdev. I think the two type-relation checks which are adapted
here have the same problem: they previously only applied for eager
assignability checks, but they should hold for all assignability checks
(lazy / constraint-set assignability). We could add mdtests like the
following, but I'm not sure if we really want to? This just feels like
an oversight, and I don't really want to duplicate all assignability
tests (and check with `is_assignable_to` and
`is_constraint_set_assignable_to`):
```py
from typing import Any
from ty_extensions import Intersection, Not, static_assert
from ty_extensions._internal import is_assignable_to, is_constraint_set_assignable_to
static_assert(is_assignable_to(Intersection[Any, Not[int]], int))
static_assert(is_constraint_set_assignable_to(Intersection[Any, Not[int]], int)) # fails on main, succeeds on this branch
```
## Test Plan
Manually tested
