Add max-depth flag and set|unset_max_depth internals

[mcky]

As discussed in #843 this adds the flag --max-depth=N and the internals/verbs :set_max_depth N and :unset_max_depth

Demo video of the new functionality:

CleanShot.2025-03-21.at.18.59.23.mp4

I'm a rust novice and I'm not precious about this code at all, if I've gone in the wrong direction I'm happy to hop on miaou and discuss.

Notes

• The depth passed doesn't include the current folder, which I think is more intuitive
• --max-depth=0 is allowed but basically only shows the current directory, so is useless. If we want to validate this, I wasn't sure where to do so
• I couldn't find any similar test suites to extend, so have only manually tested it

Questions

• Should I add this to Conf?
• What should the short flag be? -d is taken and -m doesn't feel ideal
• Shall I add a verb shortcut? md / umd?

As an aside I noticed there's a bit of mixed indentation in panel_state.rs and running cargo fmt creates a lot of changes (that'll teach me to run it without staging my changes..), so I've tried to format it as consistently as I can

[Canop]

Should I add this to Conf?

Not for now.

What should the short flag be? -d is taken and -m doesn't feel ideal

Let's keep it without short flag.

Shall I add a verb shortcut? md / umd?

md is make directory right now. We can keep it without verb shortcut, as user can add it if desired.

[mcky]

@Canop Thanks for the feedback and patience, I've moved the check to gather_lines, swapped for a u16 and dropped the shorthand flag. Left the Cow change as yeah I'm not confident in trying to quickly swap it out here

[Canop]

It seems to work very well. I'll merge that.