Is your feature request related to a problem? Please describe.
CPD and PMD both publish a Language interface which is redundant. Using a single interface would allow sharing more infrastructure between the two programs, eg TextDocument logic (#3784).

Describe the solution you'd like
Initially described in #3782:

Another key concern of mine is to merge PMD's language and CPD's language. That is, all languages, regardless of whether cpd.Language or pmd.Language should be registered the same way. In PMD 6 we sort of have that: CPD-only languages are registered also as PMD languages but are useless. They caused problems as they needed to be filtered out manually in some places.

In PMD 7 I think we can do better than that. LanguageRegistry could be made non-static and contain a list of languages. Then you can have a LanguageRegistry instance that contains all languages that support PMD, and another that contains the CPD languages.

This needs #3918: we could have a LanguageRegistry for languages that support PMD, and another for languages that only support CPD.