Extract a phapp.yml specification

[fago]

phapp.yml is designed for to standardize interactions with PHP applications, as needed by CI tools, hosting infrastructure or developers that want a unified interface for dealing with PHP applications. That way CI tools can read and execute phapp.yml content independent of the phapp-cli.

While the spec is built for PHP applications, there is nothing PHP specific in it. -> Let's extract the specification in a separate phapp-spec repo which only contains the specification in human readable markdown files. Then phapp-cli should refer/depend on this specification.