Make Quill instances detachable and reconfigurable

[singintime]

Motivation

This PR addresses discussion #4397 and issue #4234, providing a way to apply a new configuration to an existing Quill editor. Please refer to those for further information about the reasons behind this change.

Proposed changes

• Adding a Subscriber class, acting as a proxy to the EventTarget API and keeping track of all subscriptions to event listeners originated by Quill components and modules,
• Adding getters and setters to bind Subscriber instances to Quill root elements. Subscriber instances are stored in a WeakMap, similar to what happens for Quill instances.
• Using the subscriber to replace all non-static calls of addEventListener(),
• Unsubscribing from all event listeners originated by blots when they detach(),
• Unsubscribing from all events originated by a Quill instance when detached or reconfigured,
• Adding a detach() cleanup method to Quill instances,
• Adding a configure() method to apply a new configuration to an already existing instance of Quill,
• Providing unit test coverage for the above-mentioned changes.