Add py.typed marker for downstream type checking

[dannywillems]

Problem

The project runs mypy in CI but does not ship a py.typed marker file (PEP 561). Downstream consumers will not get type checking benefits from this package.

Notes

The serde library metaclass makes standard type annotations tricky — class-level field descriptors (fields.Str()) do not produce real type hints. The mypy config already disables several error codes to work around this. A py.typed marker would signal to consumers that types are available, even if partial.

Proposal

• Add l9format/py.typed (empty file)
• Include it in the package data in pyproject.toml