support defining different categories of types

in some ways, similar to `Not` types, but at the declaration site:
```py
@category
class Missing:
....

a: object = Missing() # error: "Missing" is in a different category to `object`, despite inheriting from it at runtime
```

The issue proposes declaration-site type categories using a `@category` decorator and gives a typing example, but it names no files, tests, or entry points. Start by locating the type-checking logic for `Not` types and determine how category declarations and assignment errors should be specified before implementation.

Written by the indexing model from the issue text.