Tab inline completion

[dandavison]

This PR makes nushell tab completion behave more like zsh/bash. Assuming quick_completions = true:

• Changes Tab so that it completes instead of selecting the next menu item (arrow keys can be used for that)
• Completion is "as far as possible". So, the entire word if there's a unique completion, or else up to the shared prefix of possible completions.
• Gets rid of the quick completion behavior whereby a word was completed automatically as soon as the input came to have a unique completion.

See nushell/nushell#6802 to use this with nushell.

[fdncred]

I'm guessing we need to land this one so we don't have to point to @dandavison's repo?

[sholderbach]

Sounds good!