feat: Dis/Allow -- as Value

[mkovaxx]

Fixes #6200.

TODO:

• update docs

[epage]

Note that we generally ask for discussing the general solution on the Issue and getting agreement on that before we will look at PRs, see https://github.com/clap-rs/clap/blob/master/CONTRIBUTING.md#where-to-start

[mkovaxx]

Note that we generally ask for discussing the general solution on the Issue and getting agreement on that before we will look at PRs, see https://github.com/clap-rs/clap/blob/master/CONTRIBUTING.md#where-to-start

@epage Got it, thanks for the guidance here!

Added a proposal outlining the general approach and rationale on the original issue:
#6200 (comment)