Add the user-interface independent part of a generic script interpreter

Fiji introduced interpreters for the script languages it supported. When ImageJ2 overhauled the idea of Fiji's scripting framework, reimplementing it cleanly on top of the SciJava common framework, the interpreters were dropped due to a misconception how widely in use they are. For example, Pavel Tomancak – the father of Fiji – pointed out that he uses the interpreter quite heavily.

Therefore, let's introduce a clean rewrite of the script interpreters. This topic branch adds the backend that is not visible to users; a corresponding Pull Request will be opened in the `imagej-ui-swing` project in a moment.