Support for mamba init fish

[dlukes]

See #1718 for details.

[jonashaag]

LGTM. I wonder if we can somehow deduplicate the function micromamba code with the stuff in mamba.fish

[dlukes]

I wonder if we can somehow deduplicate the function micromamba code with the stuff in mamba.fish

Yeah, I thought about that too. They'd probably have to be generated from a template in a build step. Note it's not just the name that's different, one invokes just $MAMBA_EXE, the other one alternates between $MAMBA_EXE and $CONDA_EXE.

But then I thought -- realistically, these are unlikely to change often, so probably not worth the added complexity, and better keep it simple.

Let me know if you think otherwise though.

[wolfv]

Yeah, I think it's not worth the effort right now :) Thanks for making the PR! Now we just need the same for Powershell (#1088)