Remaing tasks:

[ ] The prototype currently is currently structured such that every single pass would be its own plugin. Instead, we should let plugins return a container of passes.
[ ] We need to define a directory from which the plugins are loaded.
[ ] We need a test plugin to verify that loading works.
 
After the three sub-tasks are done, the framework should be in a mergeable state. Implementing actual useful passes will be done in separate tasks – they might (re-)shape the API, but we can do those iterations once the framework is merged.

Notably, we don't want to expose QQmlJSScope as public API, but we can still address that at a later point (by using thin facadce classes).