Split experimental access modes into submodules

Problem/Motivation

Currently all access modes are part of the main module. While especially the "Permission-based" access mode seems quite stable or can become stable, the inheritance access modes are a lot more complicated and experimental and require the entity_usage module dependency.

That way the module can be made "stable" and the experimental inheritance modes with their dependencies can be separated.

So with only the main module enabled, only "No specific restriction" and "Permission-based" should be available here:

With the inheritance submodule enabled, all four options should be made available.

Steps to reproduce

Proposed resolution

1. Create a 2.x branch to implement this
2. Implement a hook to register additional restriction modes for the settings form
3. Move the inheritance based implementation into a submodule, together with the entity_usage module dependency and mark the submodule experimental
4. Write access tests for 2.x
5. Release 2.0.0

Remaining tasks

User interface changes

API changes

Data model changes