Support for env and secret files

[simondotm]

Add support for function environment vars. Solution:

• Firebase app generator adds environment folder
• Function generator provides default empty .env, .env.local and .secret.local files in this dir
• Add glob assets rule to functions project.json to copy them from parent firebase app environment folder when functions are built
• Update sync generator to auto-rename the glob assets input dir rule if parent firebase app is renamed
• Update getconfig target to place .runtimeconfig.json in the firebase app environment dir
• Update functions in firebase.json to have ignore: [ "*.local" ] rule to prevent local files being deployed

Outcome is that functions get the common environment vars copied from the parent firebase app to their dist folder, which allows:

1. .env* files to be deployed with function code
2. *.local and .runtimeconfig.json files to be accessible to firebase emulator when serving locally

Documentation updates also in this PR.

[Gurey]

Great stuff and great work!

We really like this project at our company and just started to use it.

Do you have any idea when this will be released?

[simondotm]

@Gurey thanks for the kind feedback. Hopefully in next week or so, I'm just finishing up some migration tooling.