Soft deletes, polymorphic to-many and multi-resource models

Added

• #43 The package now supports soft-deleting resources. For full details on how to apply this to resource schemas, refer to the new Soft Deleting chapter in the documentation.
• Multi-resource models are now supported. This allows developers to represent a single model class as multiple different JSON:API resource types within an API. Refer to documentation for details of how to implement.
• #8 The new MorphToMany relation field can now be used to add polymorphic to-many relations to a schema. Refer to documentation for details.
• Developers can now type-hint dependencies in their server's serving() method.
• Can now manually register request, query and collection query classes using the RequestResolver::registerRequest(), RequestResolver::registerQuery() and RequestResolver::registerCollectionQuery() static methods.