Improve colorchoice enum

[pksunkara]

This is done because of the decisions made in discussion for colorchoice_clap crate, see rust-cli/anstyle#102.

ColorChoice::possible_values is a convenience for builder users.

As for ValueEnum::to_possible_value, it was originally written this way to mimic ripgrep's --help output. However, this forces short/long help distinction when there might not be otherwise. Looking at other programs, like git, there doesn't seem to be a longer description of the values (an "always ansi" makes things more complicated).

[epage]

Thanks!