Meson: add sage cli

[tobiasdiez]

Meson currently doesn't install any of the scripts in src/bin. This is because

• they mostly don't make sense for meson - especially doctests, sage packages interactions and various other developer tools shouldn't be installed for normal users
• they rely on tricky environment variable manipulation
• they use bash and thus do not work on Windows

Here, we reimplement a very small subset of the sage cli functionality in Python without any env hacks. At the moment only --version and the interactive sage shell are implemented.

What would be other command that make sense in the context of meson? We might also want to use the chance and reconsider the naming/layout of some of the commands.

:memo: Checklist

• [ ] The title is concise and informative.
• [ ] The description explains in detail what this PR is about.
• [ ] I have linked a relevant issue or discussion.
• [ ] I have created tests covering the changes.
• [ ] I have updated the documentation and checked the documentation preview.

:hourglass: Dependencies

[antonio-rojas]

I'd say sage -n to run the notebook directly and sage -c to run a single command should definitely be kept. I personally use sage -t a lot for running tests (I know it can be replaced with python -m sage.doctest, but the shortcut is quite handy)