feat: make Equatable `abstract mixin class`

[utamori]

Status

READY

Breaking Changes

NO

Description

Make Equatable class an abstract mixin class so that it can be used as a mixin

Related PRs

no

Todos

• [x] Tests
• [x] Documentation
• [x] Examples

Steps to Test or Reproduce

Outline the steps to test or reproduce the PR here.

dart test

Impact to Remaining Code Base

This PR will affect:

• Probably not.