Improve __init__.pyi and _typing_utils.pyi typings - #1055

Multiple changes:

1. Style: None must be the last item in a union
2. Using Final for constants. It changes the inferce:

Before:

» mypy ex.py
ex.py:3: note: Revealed type is "msgspec.UnsetType"
Success: no issues found in 1 source file

After:

» mypy ex.py
ex.py:3: note: Revealed type is "Literal[msgspec.UnsetType.UNSET]?"
Success: no issues found in 1 source file

3. Using TypeIs instead of TypeGuard, so we can better narrow in the negative case as well. This check is symmetric.
4. Renaming T -> _T to remove type-checking name from possible imports.
5. Fixing bases type, it can accept all types, not just structs