Add mypy typing support

[s4m0r4m4]

Per PEP 561 (https://peps.python.org/pep-0561/), adding a py.typed file will indicate to consuming code that this library includes type hints. A brief survey of the code indicates this should work ok.

If approved, I could potentially add the py.typed marker file.

[my1e5]

Yes, I can reproduce this

# main.py
import tsdownsample

print(tsdownsample.__version__)

$ uv run mypy --version
mypy 1.18.2 (compiled: yes)

$ uv run mypy .
main.py:1: error: Skipping analyzing "tsdownsample": module is installed, but missing library stubs or py.typed marker  [import-untyped]
main.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

After adding a py.typed file:

$ uv run mypy .
Success: no issues found in 1 source file