pmd extension descriptor draft

Java.xml

<!-- xmlns and stuff -->
<language id="java" displayName="Java">

  <versions>
     <version id="13" displayName="Java 13" />
     <version id="15" displayName="Java 15" />
  </versions>

  <parser class="...JavaParser"/>
  <violationSuppressor class="...JavaViolationSuppressor"/>

  <cpdTokenizer class="...JavaTokenizer">

  <builtin-rulesets>
     <ruleset ref="classpath://category/java/errorprone.xml">
  </builtin-rulesets>
</language>