Prepare for a fancy ImageJ 1.x macro interpreter

We are about to add an ImageJ 1.x macro "script engine". By necessity, it will be *very* limited, but we can still make things a little bit better by handling `@Parameter` comments via the `ScriptService` (see imagej/imagej-legacy#10). This will require a little change in the `ScriptService`, though: we will need to tell what inputs and outputs are expected when evaluating the script, to be able to insert appropriate code before and after the script.