ui: add color to help text

[gechr]

I appreciate that this might not be to everyone's taste, but I find help text output more readable with colours.

I chose the default clap v3 colours since that's what I've become used to with Rust tools, but happy to tweak as desired, or close this PR entirely if the plain appearance is the jj maintainers' preference.

Before

After

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[martinvonz]

I personally like the colored styled. Can you remind me why clap4 switched away from it?

[gechr]

@martinvonz There's no strong justification as far as I can tell beyond "polishing up" - clap-rs/clap#4132

[martinvonz]

Thanks. I think that's quite well justified, actually. For reference, here's what it says, with some comments from me interleaved:

The greens and yellows are gone, instead using bold, underline, and dimmed: PR 4117 though some more work might be needed to enable colored help, depending on the needs

• See PR for screenshot
• Dimmed was removed from placeholders in fix(help): Don't dim placeholders #4126
• Some liked the colors (example), some hated them (example)

We use a lot of colors too and some people hate it. So I don't think this argument applies to jj. (We may want to define some kinds of "color scheme" or way of resetting colors without disabling them, but that's mostly unrelated to this PR.)

• Color makes it easier to scan

+1

• Some applications have specific branding that it clashed with (Make colored help output more compatible with an applications overall theme #2963)
• True colors would be prettier but requires getting into terminal theme detection to make sure it plays nice and hesistant to do that at the moment
• Users can't rollback until theming support is in (Allow good/warning/error/hint color to be customized #3234) though they can just disable the coloring with Command::disable_colored_help(true)

These are a bit relevant to us. I don't think the default clap colors conflict much with our color scheme. I don't think users are going to interpret the green commands as good and the yellow headings as warnings, for example. It seems reasonable for us to let the user define the colors in [colors] (see https://jj-vcs.github.io/jj/latest/config/#custom-colors-and-styles), however. I don't think that needs to be part of this PR.

So this PR looks good to me, but I'm not going to approve it yet because I think we should give others a chance to comment first.

[yuja]

I don't have strong feeling against the clap's default nor this PR.