Add public API typing stubs

[aviadr1]

Summary

• extract the useful typing/API ergonomics idea from PR Add better portability to uvloop and winloop #1 into the modernized src-layout package
• add a public init.pyi stub while keeping runtime names as direct backend aliases
• document typing behavior and bump version to 0.2.2

Why a .pyi stub instead of runtime Protocol/ABC wrappers

The runtime objects are intentionally direct aliases to uvloop or winloop objects. A .pyi stub improves editor and type-checker behavior without introducing wrapper classes that could alter identity checks or backend behavior.

Validation

• uv lock --check
• uv run ruff check .
• uv run pytest
• uv build
• uv run twine check dist/*
• verified the wheel contains winuvloop/init.pyi and winuvloop/py.typed

Closes #1