Add staticModifiers flag (to support shaking unused modifiers and route splitting)

[simonihmig]

AppFiles does not handle modifiers, they are all put under otherAppFiles. I believe they should be treated similar to helpers, so they get split into route bundles, right?

[ef4]

Yes, and not just for route splitting. This also enables tree-shaking of unused modifiers.

To make this work, we need to add a staticModifiers flag that works like staticHelpers. The flag must do two jobs:

1. It stops us from importing all the modifiers directly from the engine entrypoint (typically assets/your-app.js). It would work like staticHelpers does here. This depends also, as you already guessed, on changing AppFiles so it keeps modifiers in their own list, separate from otherAppFiles.

2. It causes us to do build-time resolution of all modifiers that appear in templates. This would be implemented in the resolver transform and probably add to the CompatResolver like here.

The tests for this feature would go in resolver tests. You'd be testing to make sure that we find the modifier as a dependency of any template that uses it. And also that we throw a good error if you try to use a modifier we can't find (when staticModifiers is enabled).