Pixi tab completion should include runnable tasks under pixi run <tab>

[jleibs]

Problem description

Good tab completion is an important part of both productivity and discoverabillity in task-runners. The fact that we can't tab complete on runnable tasks makes this a harder sell on migrating over from just.

Unfortunately this doesn't come for free out of clap (yet... clap-rs/clap#1232).

I took a look at how just handles this and it looks like a mixture of clap completions with some hand-crafted patches.

See:

• https://github.com/casey/just/blob/b31be27f2da63684c75e7dad7e89461ebba78944/src/completions.rs#L169
• https://github.com/casey/just/blob/b31be27f2da63684c75e7dad7e89461ebba78944/src/subcommand.rs#L303

This same approach seems plausible (though somewhat annoying) to map to pixi.

[pavelzw]

Related to #372