[6.x] Elements service

[riasvdv]

Introduction

Ports the Elements service, extracting one more new "public" service including a Facade:

• ElementTypes

Other public services that have been extracted before:

• ElementCaches [6.x] ElementCaches #18647
• ElementActivity [6.x] Element activity #18653
• ElementRelations [6.x] Refactor Element functionality to traits #18368

Operations

The Elements service itself has been split up as well to be a public service that forwards to the following Operation services:

• ElementCanonicalChanges
• ElementDeletions
• ElementDuplicates
• ElementEagerLoader
• ElementPlaceholders
• ElementRefs
• ElementUris
• ElementWrites

These services are marked as @internal - meaning they should only be used within the Element namespace, the Elements service in particular.

There is also no Facade created for any of these services as they really are meant to only be called by the Elements service.

This split allows us to have classes that are maintainable and tests that are separated by the concerns while still having one general service to interact with them.

Authorization

All authorization methods have not been ported over. These are now handled directly by using Laravel's Gate or $user->can() through the ElementPolicy.

The legacy service registers a before callback that fires the legacy authorization events.