Remove nashorn javascript engine support

[muntis]

Nashorn is known to cause memory leaks and should not be used in validations on every save. As alternative if required GraalVM should be used.

GrallVM supports javax.script.ScriptEngineManager , it means that theoretically app should have graalvm import and get correct engine from scriptEngineFactory. But I couldn't enable es6 support.

https://www.graalvm.org/reference-manual/js/ScriptEngine/

Or as website states, new implementation should use org.graalvm.polyglot.Context, here es6 works out of the box.