Add Script CLI Entrypoint

[seldonPlan]

This PR does two things:

• add a [project.scripts] section to pyproject.toml pointing to the existing cli method. The cli method is made available as a runnable script named mistune when the project is installed (which is just syntactic sugar for python -m mistune). This makes it easier to use with tools like pipx that expect an executable (see: https://pipx.pypa.io/latest/how-pipx-works/#developing-for-pipx)

• makes typing_extensions import optional based on python version being installed to (see https://typing-extensions.readthedocs.io/en/latest/#python-version-support)

[lepture]

@seldonPlan thanks for the typing_extensions fix. But the script is not needed. You can use python -m mistune to run the script.

[lepture]

@seldonPlan tests failed