Add `py.typed` marker.

[Jammf]

Downstream packages will expect the py.typed marker to be present, which explicitly declares that a library supports typing (see relevant portion of typing spec). Pyright currently emits a reportMissingTypeStubs diagnostic due to the missing marker, assuming the rule is enabled in a downstream codebase.

This PR adds the marker to the top-level penzai package, which also recursively declares that all sub-packages support type checking.