Optional requiring env var to exist

[Distortedlogic]

Currently I have some variables in my local dev env that I do not use in prod. Currently I manually change after running gen-env-types, If there was a way to set whether or not the requirement that a variable exist on a variable by variable basis, thatd be pretty sweet.

[daniellwdb]

Do you have an example input and expected output? Having a hard time understanding

[malinskibeniamin]

If I understood correctly, I believe my PR addresses your issue, feel free to take a look #35