amplify env list --json should indicate current environment

**Is your feature request related to a problem? Please describe.**
I'm writing a script that needs to know the current env name.

**Describe the solution you'd like**
The script would run `amplify env list --json` and from there easily parse the json to get current env.

**Describe alternatives you've considered**
Will have to instead rely on `amplify env list` and look for the `*` through regex. Or perhaps this info is stored in a file somewhere in amplify/backend dir?