feat: add .env support

[sirnicolaz]

Firebase supports the injection of the .env configuration, as documented here.

This turns out to be particularly useful in the context of Github Actions when for instance one would like to store all secrets in Github and pass them on to the firebase deploy script, or simply if willing to re-use the env from the Github Action inside the app.

In such a case, it's just enough to copy the env inside a .env file and the firebase deploy script will pick it up and make it visible inside the app.

This PR adds support for such behaviour.

[w9jds]

This doesn't work, you can't copy something that doesn't exist if the user doesn't specify it (as optional)

[sirnicolaz]

you are right. for some reason, when I ran it locally with the glob, it wasn't throwing an error in case of a missing file. I will check what's going on and update the pr. apologies for the inconvenience

[densk1]

I've an issue with functions .env files not being imported at build time. I'm trying to set region from .env which works for CLI deploy but not in a Github Workflow. So this would be helpful alright.

Worth noting that docs seem to indicate (in a roundabout way) that .env.* for functions should live in the /functions directory.
https://firebase.google.com/docs/functions/config-env#params