Add Apache Commons JEXL support

[duemir]

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

[jgreffe]

Hello,
do we have any idea of the purpose of fork https://github.com/jenkinsci/jexl compared to upstream https://github.com/apache/commons-jexl ?

I see only these additional commits on upstream (not part of fork) when comparing sources between https://github.com/apache/commons-jexl/tree/1.x and https://github.com/jenkinsci/jexl
apache/commons-jexl@107d93b
apache/commons-jexl@0664d10

Or is the fork patched branch the one to rely on, and in this case there are more differences.

[duemir]

@jgreffe, I have no idea. Some old-time core developers might have some thoughts about it.