feat: only allow secrets to be separated by a newline

[jamesoneill997]

BREAKING CHANGE: having multiple secrets separated by spaces is now invalid syntax

Having a secret containing a space " " breaks this action. In my opinion, it's simpler to just require secrets to be split be on their own line, having multiple secrets on the same line is also a little hard on the eyes.

I've smoke tested this on the offending repo that it was breaking for me on, pointing the action to my fork and putting secrets on new lines fixed the issue, the deployment succeeded with correct secrets.

This would be a breaking change for existing workflows, so will require at the very least a minor bump.

[jamesoneill997]

Not sure who I should tag for a review as there doesn't seem to be a contribution entry in the readme, the last person I can see to have merge access is @jphenow , so I'm just wondering if you wouldn't mind reviewing this if you get a chance, please :)? My apologies if I've tagged the wrong person!