Types issues in headings

[Witoso]

📝 Provide a description of the improvement

Taken from #15785

Type definitions to support custom heading models

Currently the typescript definition for heading configs, uses this union type:

ckeditor5/packages/ckeditor5-heading/src/headingconfig.ts

Line 75 in 89c1831

export type HeadingOption = HeadingElementOption | HeadingParagraphOption;

These options explicitly specify heading1-6, and paragraph models. However, according to the docs, we're able to specify our own models. (Docs example is headingFancy).

ckeditor5/packages/ckeditor5-heading/src/headingconfig.ts

Line 82 in 89c1831

model: 'heading1' | 'heading2' | 'heading3' | 'heading4' | 'heading5' | 'heading6';

ckeditor5/packages/ckeditor5-heading/src/headingconfig.ts

Line 110 in 89c1831

model: 'paragraph';

Could we add some HeadingCustomOption type to the union so we don't have to use @ts-ignore?

UpcastAlso in typescript definitions?

I believe the heading options support upcastAlso as a property. For example in heading tests:

ckeditor5/packages/ckeditor5-heading/tests/headingediting.js

Lines 93 to 106 in 89c1831

	heading: {
	options: [
	{ model: 'paragraph', title: 'paragraph' },
	{
	model: 'heading1',
	view: 'h1',
	upcastAlso: [
	{ name: 'p', attributes: { 'data-heading': 'h1' } }
	],
	title: 'User H1',
	converterPriority: 'high'
	}
	]
	}

Can this be added to the typescript definitions for heading options?