Support querying environment variable presence/absence

[Gabriella439]

Fixes #452

[9999years]

Looks good! Two comments:

1. The names don't include env or anything like it, which is a little ambiguous, especially because they take a Text. I could imagine reading isPresent "foo" and thinking it's testing if a file exists.
2. I think isSet and isUnset are potential footguns — in my experience, it's very rare to consider a variable 'set' if it exists but is set to the empty string. We might consider adding a note to this effect to the documentation. ("In most cases, you want isPresent.)

[Gabriella439]

I'm happy to add an "env" or something to the names to address (1) so I'll do that.

I'm a bit surprised about (2) because I had the opposite point of view (that isPresent / isAbsent were the footguns). The reason why is that treating an in-band value (the empty value) as a sentinel value that means unset is a common source of programming errors (not just in Bash, but in general). Like, in the case of a simple binary toggle this is less of a concern, but when the environment value stores an actual payload that the program will use then there is a greater risk that the payload can be meaningfully present, but empty.

[9999years]

Hmm, that's true. In my experience most environment variables are used for simple toggles. I would also expect the program to do some validation if the environment variable has semantics (e.g. check that a path exists if it's supposed to represent a path, etc.).