feat: add support for fossil-scm in prompt

[nat-418]

Description

Add fish prompt function and include it in the vcs prompt function.

Fixes issue #9497

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[faho]

1. The in-repository will fail in subdirectories
2. Some style nits

[faho]

As a general question: How long does this take inside of a repo with fossil installed?

You can check with time fish_fossil_prompt.

[nat-418]

As a general question: How long does this take inside of a repo with fossil installed?

You can check with time fish_fossil_prompt.

$ time fish_fossil_prompt
Executed in   84.28 millis    fish           external
   usr time   13.72 millis    3.16 millis   10.56 millis
   sys time    9.85 millis    0.01 millis    9.84 millis

[nat-418]

1. The in-repository will fail in subdirectories

   2. Some style nits

Thanks for the quick and helpful feedback. I apologize for forgetting to mark this PR as a draft when I opened it.

[faho]

Merged, thanks!

For the record fossil ls outside of a fossil repo takes ~3-4ms, so it's about as fast as git. We could use builtins here like hg, but it's a much different trade-off (hg status is 150ms outside of a repo, hot cache).

[nat-418]

Thank you! I love fish!