Include better error message when extending from Equatable

**Is your feature request related to a problem? Please describe.**
Whenever a model extends a class from an external package it is not possible to add `@ignore` on its members. For example, this happens when extending from `Equatable`, it even has its own dedicated discussion https://github.com/isar/isar/discussions/648.

When running the isar_generator the error messages are quite handy but suggesting to add `@Collection(inheritance: false)` (or equivalent) is not included within the error message.

**Describe the solution you'd like**
As a developer, I would like the isar_generator to inform me to include `@Collection(inheritance: false)` whenever I'm inheriting from an external library (or any other case where adding `@ignore` in the super's member is not possible).

**Version**
- Platform: N/A
- Flutter version: 3.13.7
- Isar version: ^3.1.0+1

The issue does not name a file or test; begin by locating isar_generator's existing diagnostic for inherited members and how it handles @Collection(inheritance: false). Confirm the message explains this option when @ignore cannot be added to a superclass member, then run the generator's relevant tests or reproduction to verify the guidance.

Written by the indexing model from the issue text.