CompilerProvider: consider allowing to claim multiple file extensions

[evacchi]

CompilerProviders can only claim one file extension, but some compilers may process more than one file types. This can be circumvented by creating one abstract class with the main logic, and then overriding handledExtension() in the subclasses, but it might be limiting.

Proposal: allow to return multiple handled extensions, or use a predicate of the form boolean handlesExtension(String ext)