h4, h5, and h6 fail to downcast GHS added attributes

[lauriii]

📝 Provide detailed reproduction steps (if any)

1. Enable html-support, source-editing, and headings plugins
2. Configure GHS to allow all elements and attributes
3. Configure headings plugins with the example from https://ckeditor.com/docs/ckeditor5/latest/features/headings.html#configuring-toolbar-buttons
4. Add a h2 element via the headings plugin.
5. Enter source mode and give it a class of "test".
6. Exit source mode.
7. Via the headings plugin, change the h2 to an h3.
8. Enter source mode and observe that the class "test" is still there.
9. Exit source mode.
10. Via the headings plugin, change the h3 to an h4.
11. Enter source mode and observe that the class "test" is gone!.
12. Exit source mode.
13. Via the headings plugin, change the h4 to an h3.
14. Enter source mode.
15. Observe that the class "test" is now back!

✔️ Expected result

class attribute should downcast on the heading regardless of its level.

❌ Actual result

class attribute doesn't downcast on h4, h5, and h6.

❓ Possible solution

It seems like https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-html-support/src/schemadefinitions.js maps heading views to elements. However, not all supported heading views are mapped. Also, the mapping assumes that heading1 is mapped to h2 while its mapping can be configured.

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[Reinmar]

We need to remove this:

ckeditor5/packages/ckeditor5-html-support/src/schemadefinitions.js

Lines 54 to 65 in 1c0dace

	{
	model: 'heading1',
	view: 'h2'
	},
	{
	model: 'heading2',
	view: 'h3'
	},
	{
	model: 'heading3',
	view: 'h4'
	},

And add GHS's integration with the Heading feature, because the list of headings that it supports is configurable via config.heading.items. The model and view element names are fully configurable there, so a static definition in schemadefinitions.js makes no sense.

Flow:

1. Introduce HeadingIntegration plugin (see src/integrations/).
2. Check whether Heading is enabled. Abort if not.
3. Read config.heading.options (ignore potential validation/normalization of invalid config)
4. Add these options via DataSchema.registerBlockElement() with just the model and view properties
5. Remove the existing heading1-heading3 definitions from schemadefinitions.js.
6. Add HeadingIntegration in GeneralHtmlSupport's requirements.

Scope:

• Solution
• Tests