Migrate script language plugins to external repos

See:
* https://github.com/scijava/scripting-beanshell
* https://github.com/scijava/scripting-clojure
* https://github.com/scijava/scripting-java
* https://github.com/scijava/scripting-javascript
* https://github.com/scijava/scripting-jruby
* https://github.com/scijava/scripting-jython