Nushell: Support partial completions

[ryanpcmcquen]

Coming from fish shell, I miss the partial completion with Alt and arrow keys.

Here's an example of partial completion in action. I find this feature infinitely useful!

Perhaps this could be part of the convo in this issue as well:
https://github.com/nushell/nushell/issues/290

[fdncred]

I would love to see this too.

[thegedge]

☝️ Removed the "completions" label, which I've been using to mean "completion suggestions". This is what we call "hinting", where we hint to what the user wants the line to be, based on historical lines. A little pedantic, but this helps me focus my work around completions 🙂

[thegedge]

I think this is missing functionality in rustyline. If we take a look at https://docs.rs/rustyline/6.2.0/rustyline/enum.Cmd.html, the only option we have is CompleteHint. In other words, we can complete the entirety of the hint, but not partially. We'll either need rustyline to add:

1. partial hint completion, or
2. custom commands for key bindings.