A cli prompt selector

[faho]

It's a bit weird to have to fire up a browser to get fish_config to
choose a prompt.

So this adds prompt:

• prompt list shows all the available prompt names
• prompt show demos the available sample prompts
• prompt choose sources a prompt
• prompt save makes the choice permanent

Part of #3625.

This means that you can e.g. pick the "informative" prompt by adding prompt choose informative to config.fish, and it will just always source the one from the currently installed fish (so it will update when we update the sample). This is on purpose and has basically no overhead, so it's entirely acceptable - we might even want to recommend it in the docs.

TODOs:

• This does not currently correctly show the right prompt, which is awkward
  to do because we'd have to move it to the right (and this involves width-without-color, see string pad doesn't work with escapes #7784).
• How about the name? prompt, or maybe a more general "config tool"? A special mode for fish_config, so if you do fish_config prompt it'll run in the terminal, fish_config will keep opening the browser?
• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[zanchey]

This is rad.

Name: There's only one package in Debian that has a /usr/bin/prompt command, and it's libmodglue1 which is installed on a tiny fraction of reporting Debian installs, so I don't think prompt is bad as such... but I wonder if fish_choose_prompt or something would be better. Alternatively, I do like the idea of fish_config being able to do both terminal and Web configuration, with autodetection (eg for SSH) and an appropriate override flag.

UI: I think a bare prompt should be the same as prompt list. I think prompt choose should probably imply prompt save.

[faho]

I think prompt choose should probably imply prompt save.

The idea here is that you can use prompt choose to test-drive a prompt - you can do prompt choose scales, then cd around a bit, run a few commands, see what it does.

Or, like I added in above, you can just add prompt choose scales in config.fish and it will track the updates to the sample (this would make #7918 nicer because it would allow us to remove the now-useless function!).

We could add a prompt choose --save shorthand (like funced --save), but I do believe not saving by default is correct, because test-driving is a very normal thing to do.

[faho]

(note: This was added in preparation of moving the sample_prompts directory out of web_config - because it no longer is web-exclusive)

[faho]

I think a bare prompt should be the same as prompt list.

Done

[faho]

Current output:

[faho]

Alright, moved it over to fish_config. It's now fish_config prompt <subcommand>.

The web-ui can be accessed via fish_config or fish_config browse. Selecting the initial tab (which is a feature I doubt many people use or know about) can now only be done with fish_config browse.

[faho]

Alright, selecting the initial tab is broken anyway (even without this). I'm just gonna remove it. Fixed it, turns out the URL changed at some point.

[faho]

Alright, merged.