Add PHP version as optional action input

[braindawg]

Psalm allows us to specify the PHP version it analyzes code against. Exposing this option allows users of this action to supply a matrix of PHP options to enable Psalm runs against each of the PHP versions they would like to support, or just to analyze against a specific version of PHP that differs from the container's installed version.

This provides a partial solution for #60, as the container-installed PHP version would no longer limit the versions Psalm could analyze against.