Allow standalone Twirl usage

[oxc]

Since Twirl is (for quite a while now) available as a standalone library, it would be nice if the plugin would support standalone usage, without requiring play libraries to be present on the classpath.

The main issue seem to be the default imports that cannot be overridden.

I managed to get the plugin to a somewhat usable state for standalone Twirl by providing mock objects for those imports, but of course that's an ugly workaround, especially since new imports keep getting added (for example #259).

[oxc]

In fact Twirl has been working on making its own imports even more flexible (see for example playframework/twirl#71), so it would be nice if the IDE wouldn't stand in the way.