Support FORCE_COLOR

[nhurden]

https://force-color.org/

Fixes #1305

Also updates NO_COLOR to handle empty strings correctly per the spec:

Command-line software which adds ANSI color to its output by default should check for a NO_COLOR environment variable that, when present and not an empty string (regardless of its value), prevents the addition of ANSI color.

[Delgan]

Thanks for the new implementation of FORCE_COLOR and the fix of the inaccurate implementation of NO_COLOR. 👍

There appear to be many competing standards, see also: CLICOLOR.
Initially, I had reservations about which of these variables should be honored. In particular because I've read various OSS maintainers complaining about the situation and refusing to implement some of them.

However, I just discovered that the Python CLI itself respects NO_COLOR and FORCE_COLOR, which makes it the de facto standard to follow for Loguru as well.