[ty] Split `ty_extensions` into public and internal parts (#26586)
## Summary
Split `ty_extensions` into a public part (`ty_extensions`), and an
internal part (`ty_extensions._internal`). For now, move everything to
`_internal`, except for:
- `static_assert`
- `Unknown`
- `AlwaysTruthy`, `AlwaysFalsy`
- `Intersection`, `Not`
- `Top`, `Bottom`
- `JustFloat`, `JustComplex`
- `NamedTupleLike`
