[FEATURE] Please add py.typed for mypy

To use mypy in strict mode you need to make sure that 3rd party libraries are marked as typed. to do this it's just a matter to add an empty `py.typed` file in the root package

**Is your feature request related to a problem? Please describe.**
I want to be able to run `mypy` and not having
`error: Skipping analyzing "fasthtml.common": module is installed, but missing library stubs or py.typed marker [import-untyped]`

**Describe the solution you'd like**
just add a `py.typed` file in the package

Locate the package root containing fasthtml.common and inspect the packaging metadata to see which files are included. Add the empty py.typed marker there, then run mypy in strict mode against an installed build to confirm the import-untyped error is gone.

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