Extract private `_concatStyles` hook to allow customized concat strategies.
Extract private `_concatStyles` hook to allow customized concat strategies.

This is private for now, because I suspect that we should actually deprecate
this "auto concat all the things" behavior completely (in favor of the preprocessor
in use emitting the correct file).

ember-engines/ember-engines,dgeb/ember-engines,dgeb/ember-engines,ember-engines/ember-engines