feat: fish shell completion for tsh

[SMillerDev]

This adds some basic shell completion for tsh, mostly for tsh ssh suggesting clusters/nodes and tsh ls suggesting labels.

Related: #3753, #6933, #7170

[russjones]

@codingllama You're a fish user right? Do you mind taking a look?

[SMillerDev]

Fixed some issues I had with scp not completing files correctly

[russjones]

@SMillerDev Thank you for the contribution. Unfortunately we won't be able to merge.

The CLI flag parsing library we use kingpin supports generation of full autocompletion scripts for bash and zsh. We recently enabled that for tsh.

• Update kingpin & allow autocompletion #26238
• Add docs for shell completion #26596

If we add this, we would have two different ways to consume autocompletion and they would be in differing states of completeness. It would be difficult to document and confusing for users. In the face of that, I think it's better we not merge.

There is a issue in the kingpin to add support for fish. Once that is implemented, it should be a simple version bump to add fish autocompletion to tsh.

• Feature Request: Fish completions alecthomas/kingpin#259