Add stubs file to PyAV

[RuoXian233]

Overview

There's currently no .pyi files in the PyAV installation path, which brings much difficulty to the development, as the language server
couldn't resolve any symbols and provides no completion or hints.

Desired Behavior

Add some basic stubs to PyAV, I'm not looking forward to a detailed type hint, just function signature and some documents,
I'd heard that there's some tools like stubgen has the ability to finish the task automatically, but I'm not sure that it will work with Cython files and C module.

Additional context

I'll also try to write the stubs by myself, but I certainly need more help

[HanzCEO]

Hello @RuoXian233,

Would you like to draft a new PR and work on it? I would like to help.

[github-actions]

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