Prompt user if extant .env file would be overwritten by running env-init

[Lunaetic]

PR replaces a TODO: with bash scripting that prompts the user to confirm that .env should be overwritten.

Testing

• Pull PR
• Create a new, empty directory, and from that directory, run warden env-init test magento2; assert that the .env file is created as normal
• Make a change to the .env file
• Run warden env-init test magento2 again; when prompted, answer n; assert that the change you made previously is still in the file
• Run warden env-init test magento2 again; when prompted, answer y; assert that the change you made previously is no longer there
• Run warden env-init test magento2 again; when prompted, answer anything other than y or n; assert that you are asked to provide a valid answer