Refactor plugin system to use DI instead of new operators

I just wanted to give a quick heads up that I’m going to do going to see how far I can take https://github.com/eclipse-theia/theia/issues/5761 this sprint. I’m going to start with doing similar refactorings like https://github.com/eclipse-theia/theia/pull/6148 and then go from there and see what we can do.

[original thread by Josh Pinkney]