[Feature] Expose --color to as variable/function

It would be nice to allow recipes to query if `just` output is colored or not, recipes could use color whenever just output itself is colored.

Example (syntax for demonstration only):
```justfile
color := if colored_output() == "true" {"always"} else {"never"}

compile:
gcc --color="{{ color }}" foo.c
```

A justfile like this could be nice in CI to color subcommands (call via `just --color=always`), and still behave correctly when piped to a file.

As far as I see `just` can independently color its standard output and error streams, so it might make sense to provide queries for both.