Enhance the BuildContext with the produced output file

I'm writing a `CompilationParticipant` and noticed that the `org.eclipse.jdt.core.compiler.BuildContext` has a reference to the _source file_, but not to the location where the compiled _class file_ will reside.

It would be great to have a `BuildContext#getClassFile()` method (that may returns null untill the source is really compiled), see also
- https://github.com/eclipse-jdt/eclipse.jdt.core/discussions/669