Warn if ExecuteAlways is applied to class with wrong base type

`[ExecuteAlways]` can be applied to `MonoBehaviour` based classes, so is a good candidate for the `[BaseTypeRequired]` annotation. However, it can also be applied to `SystemBase` classes.

`BaseTypeRequired` can only suggest a single base class (with multiple interfaces). Add a warning to suggest deriving from `MonoBehaviour` or `SystemBase`, with quick fix.

Note:
> ComponentSystemBase would be a better test since that would also cover ComponentSystem and JobComponentSystem variants

Locate the inspection and quick-fix handling for ExecuteAlways and the existing BaseTypeRequired annotation support. Use ComponentSystemBase as the test case, covering MonoBehaviour and SystemBase alternatives; done means an incorrect base type produces a warning with a quick fix suggesting either valid base class.

Written by the indexing model from the issue text.