feat(playground): register Header tool

[Reversean]

Registers @editorjs/header in the playground.

Getting Header to actually render surfaced two bugs, both fixed here.

• BlockManager.insert spread a block's data onto the block object's own top-level properties instead of nesting it under data, so every tool lost its data on insert.
• ToolboxUI.addTool read tool.options.toolbox and added a single popover entry per tool; it now iterates tool.toolbox, since a tool can define several entries (Header has one per heading level) and only the first ever showed up.

[gohabereg]

Let's add @editorjs/editorjs pacakge inside the monorepo. Mark it private for now.

This package should import Core and all the Tools and register them. Export the core with registered tools.

Playground should use import from editorjs/editorjs package instead of core.

Registrations inside the core could be removed

[github-actions]

Unit Tests

Package	Coverage	Delta
@editorjs/core	73.14%	N/A
@editorjs/model	98.48%	N/A
@editorjs/clipboard-plugin	66.66%	N/A
@editorjs/dom-adapters	86.95%	N/A
@editorjs/ot-server	20%	N/A
@editorjs/shortcuts-plugin	100%	N/A
@editorjs/editorjs	100%	N/A

Mutation Tests

Package	Mutation score	Dashboard URL
@editorjs/core	88.89% 🟢	Dashboard
@editorjs/clipboard-plugin	No files to mutate found.
@editorjs/model	No files to mutate found.
@editorjs/dom-adapters	No files to mutate found.
@editorjs/shortcuts-plugin	No files to mutate found.