setup.py: define an entrypoint for the CLI

[nicoonoclaste]

This way, a unicodeit executable will be generated, which just stubs out to unicodeit.cli.

[nicoonoclaste]

For context, I wrote this while packaging unicodeit for Nix, as it made sense to have a unicodeit package expose a unicodeit program (so Nix tools can automatically fetch-and-run it based on its mainProgram metadata)

[ilyagr]

This PR can also be used to install unicodeit with uv as follows:

uv tool install git+https://github.com/svenkreiss/unicodeit@refs/pull/79/head

If this PR were merged and published to PyPI, uv tool install unicodeit would likely also work.

Something analogous is probably true for pipx as well.