IDEA Editor: Adds PHPStorm support

This change extends the IDEA package to support PHPStorm. This differs from the existing builds, as PHPStorm makes use of the Java port of Snappy, so no patching should be necessary.

This is my first attempt at writing a package in Nix, so I would appreciate all suggestions for how this could be improved.
Updated to make use of `optionalString` and `concatStringsSep`.