Stop using regex for validation

[edmorley]

This buildpack currently uses regex to validate Procfile contents:
https://github.com/heroku/procfile-cnb/blob/a1ee3659a5a95df96bf7b41a5f9dffb40678f4dd/src/procfile.rs#L38-L43

This causes quite a bit of buildpack bloat, particular with newer versions of the regex crate, which have gotten even larger (since by default it favours runtime performance for large/complex payloads/regex over compile time/binary size) - see #175 (comment).

The original reason for using regex was likely to ensure compatibility with the existing Codon Procfile parsing, however, (a) it would be easy to maintain that even without regex, (b) we've talked about wanting more strict parsing anyway (#73).