Implement vscode-styled variables

[cooperrunyan]

Provides some of the functionality mentioned here

[Agokir]

asmjamagdy11

[cooperrunyan]

If it doesn't start with env: or match any of the supported variables, the function returns undefined. Later on, when we are actually replacing values, it calls the resolveVariable to attempt to substitute it, and if it returns undefined (i.e. couldn't determine what the variable means), then it just leaves the whole ${...} alone.