Scripting: Add script engine for lucene expressions

Adds support for new "expression" script engine.

This is backed by lucene expressions.  These are javascript expressions, which can only access numeric fielddata, parameters, and _score.  They can only be used for searches (not document updates).

closes #6818
I am kind of thinking these should be `ExpressionScriptCompilationException` since the script isn't actually executing here, it's just in a late-stage compilation. What do you think?