WISH: File tag for testing whether an env var is set or not

[HenrikBengtsson]

It is possible to do:

• FOO=abc.R
• FOO=def.R
• FOO!=abc.R

but it is neither possible to test for empty value nor non-defined values; the following doesn't work:

• FOO=.R
• FOO=,dummy.R
• FOO='',dummy.R
• FOO!=,dummy.R
• FOO!='',dummy.R

[HenrikBengtsson]

This would allow us to test for NO_COLOR, and other environment variables whose values do not matter. What matters is only whether or not the environment variable is defined (non-empty).

In order to move forward on this, we need a syntax for testing whether or not an environment variable is nuset (or empty). It could be !NO_COLOR to check when not set, but to check when set, we can't just use NO_COLOR, because that's ambigous. Maybe !!NO_COLOR?