Add Apache Commons JEXL support

JEXL is a full expression language. It would be great to have full language support like JavaEE Expression Language or Spring EL.

This epic size. There are lots of parts to language support. The JEXL parser is defined with JavaCC which do not have any bridge to JetBrains GrammarKit so the whole grammar needs to be replicated.

https://commons.apache.org/proper/commons-jexl/ upstream
https://github.com/jenkinsci/jexl Jenkins fork
https://www.jetbrains.org/intellij/sdk/docs/reference_guide/custom_language_support.html relevant docs

Start by reviewing the Apache Commons JEXL upstream project, the Jenkins fork, and JetBrains' custom language support documentation. The issue says the JavaCC parser grammar must be replicated for GrammarKit, but it does not identify files or a staged scope. Done would require complete JEXL language support rather than a single defined change.

Written by the indexing model from the issue text.