cli/command: move prompt utilities to separate package

[thaJeztah]

See individual commits for details; may be doing some follow-ups after this.

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 67.94872% with 25 lines in your changes missing coverage. Please review.

Project coverage is 59.12%. Comparing base (8633197) to head (b74b7b3).
Report is 15 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #5952 +/- ##
==========================================
- Coverage 59.13% 59.12% -0.02% 
==========================================
Files 355 356 +1 Lines 29744 29750 +6 ==========================================
Hits 17590 17590 - Misses 11179 11186 +7 + Partials 975 974 -1 

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[crazy-max]

could we have it in pkg so we can use it in buildx repo (https://github.com/docker/buildx/blob/646df6d4a06848c675abf110dd51c2623b517590/commands/util.go#L15-L57)?

[thaJeztah]

Ah, right, but that would probably not work currently? i.e., that fork was because of docker/buildx#2359 (comment), or was that part of it reverted?

My thinking of keeping it "internal" for now at least, was because there may still be some work to do to get a stable "API" for these. There's some functions that currently depend on command.Stream (or equivalent), and ideally they'd be more generic (I'm a bit on the fence on the Stream wrappers; they're useful, but perhaps we can do without so that we can pass a regular os.StdIn / os.StdErr etc).

[thaJeztah]

Rebase was for conflict in the imports; CI is happy again, so bringing this one in