Generate WithAssertions interface with default methods

It would be good if we could generate an interface that extends org.assertj.core.api.WithAssertions and contained default methods for all the custom assertions, so that static imports are not required.

The issue names no files or tests. Start by tracing how assertj-generator emits custom assertion classes, then determine where an interface extending org.assertj.core.api.WithAssertions would be generated; done means generated output contains default methods for every custom assertion without static imports.

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