Enable customization of exit code for required env var

[johnstonmatt]

We @polyseam are happy users of Cliffy!

The .env() method is useful for denoting environment variables as "required" and providing nice error messages, but it always exits with the status code 1 and there is no way to set a more descriptive code so that users can more easily find relevant support material. This is made worse because 1 is also the default exit code by everyone else so it is quite overloaded.

I created a repo to demonstrate the above at johnstonmatt/required-env-demo.

Thanks for all your hard work on cliffy!

[c4spar]

Hi @johnstonmatt, I'm glad to hear that!

Sounds reasonable to make the exitCode configurable. Should we make a distinction between missing or invalid values?
We could also set the exitCode to 2 by default for invalid usage.

[johnstonmatt]

I think making both of those changes would be great improvements, and would definitely enable what we are hoping to do.