[ty] Type compendium (#18263)
## Summary
This is something I wrote a few months ago, and continued to update from
time to time. It was mostly written for my own education. I found a few
bugs while writing it at the time (there are still one or two TODOs in
the test assertions that are probably bugs). Our other tests are fairly
comprehensive, but they are usually structured around a certain
functionality or operation (subtyping, assignability, narrowing). The
idea here was to focus on individual *types and their properties*.
closes #197 (added `JustFloat` and `JustComplex` to `ty_extensions`).
