Support for `ty_extensions`

ty vendors a `ty_extensions` module ([src](https://github.com/astral-sh/ruff/blob/45acbd0b663b5a7ac71ed2b10fdd471948f30611/crates/ty_vendored/ty_extensions/ty_extensions.pyi)) containing very handy special-cased (by ty) types such as `Intersection`, `TypeOf`, `Not`, as well as the [optypean](https://github.com/jorenham/optype#just) `JustFloat` and `JustComplex` types.

Library maintainers (like myself) often support multiple multiple type-checkers, which in most cases limits them to whatever the typing spec allows. This means that most libraries won't be able to use `ty_extensions`, probably ever, which is a shame.

I think it would be pretty awesome if Pyrefly would have (some) support for `ty_extensions`, so that I'd be able to use it in the libraries I maintain that support only Pyrefly and ty. And because, in my experience, you're particularly open to new ideas, I thought I'd start here (i.e. before also proposing this at mypy and basedpyright).

And just to be clear; I'm not here to evangelize `ty` or something; I'm just trying to make life a bit easier, that's all :)