Fix a handful of augmentations issues. (#4376)

Fix a handful of augmentations issues.

*   Remove recommend path ordering lint. Commit to making `part` directive
    order meaningful and acceptable to rely on.
*   Allow enum declarations without values.
*   Specify signature matching for implicit setters from abstract variables.
*   Clarify that you can't augment an extension type constructor and add a body.
*   Don't allow augmenting mixin application classes.

Fix #3849.
Fix #4356.
Fix #4022.
Fix #4047.
Fix #4060.

Author: munificent