Replace `source` in environments with separate `environmentDefault` entry

[filipesilva]

Currently use use the source key inside the environments object to specify which file should be replaced:

"environments": {
  "source": "environments/environment.ts",
  "dev": "environments/environment.ts",
  "prod": "environments/environment.prod.ts"
}

But this is already the dev file. It would be cleaner to specify the default environment instead in a separate key:

"environmentSource": "environments/environment.ts",
"environments": {
  "dev": "environments/environment.ts",
  "prod": "environments/environment.prod.ts"
}

[jsanchezgarcia]

I just noticed that @filipesilva was assigned to this issue. Filipe, let me know if you want me to do any modifications to my PR (or feel free to do your own and discard mine).