Replace deprecated backbone for vanilla JS

Problem/Motivation

By the sound of #3145958: [META] Re-evaluate use of Backbone.js in core, the ancient backbone will be ditched at D11.

We need to seek for a more modern solution to replace it: likely vue, alpine, or alike.
Blazy 2.10 (DEV version as of this writing) has a plugin for object reactivity almost similar to those third parties reactive objects, only pretty inferior.
This will be enough to have vanilla JS like at GridStack:1.x while maintaining backbone heritage with very minimal reactivity.
This also has an obvious advantage to not depend on any other third party. Vanilla will do.

More info:
#2873160: Implement core management of 3rd-party FE libraries
#3198417: Official support for npm

Proposed resolution

• Replace backbone with vanilla JS

Remaining tasks

Eat enough, this is a hugo job. Do it anytime, take your time, it is a long term job, many months, likely years, before core started ditching it, don't push it too hard.

User interface changes

None, just the framework switch.

API changes

None, just the framework switch.

Data model changes

None, just the framework switch.