clap.rs colored help menus for Julia launcher

[MilesCranmer]

Currently the Julia help menu within juliaup is missing some of the juliaup-specific stuff, such as the [+channel] specifier. It also (1) lacks shell completions, (2) lacks colored output (makes it hard to read, especially with how long it has gotten), and (3) it cannot automatically wrap to terminal width.

This PR adds a full clap.rs help menu for the julia launcher. This gives us:

1. Shell completion generation for Julia across multiple shells (--generate-completions <shell>)
2. Colored hierarchy
3. Automatic text wrapping

This also adds three flags:

1. --help-raw to access the raw Julia help menu (such as for core developers).
2. --help-hidden-raw equivalent but for --help-hidden.
3. --generate-completions to generate completions for Julia using clap.rs.

(This is forked from #1234; that one needs to be merged first in any case).

Here is the current julia launcher help menu on 1.12:

And here is it on a narrow terminal:

With this PR, it looks like this:

and in a narrow terminal: