Adjust typings in headings options

[Mati365]

Suggested merge commit message (convention)

Feat (heading): Adjust typings in headings options. Closes #15790.

[arkflpc]

Why is it internal if it's used in non-internal type (line 98)?

[Mati365]

@arkflpc It should be exported and public available, but we have issue with documentation generation. Umberto incorrectly serializes union in template string literal and in API reference we have something like this:

I tried to split it between types or use it inline directly in HeadingCustomModel but it always results in [object Set] in docs.

[DawidKossowski]

I think we should change this solution a bit. Adding the new type (HeadingCustomModelSeparator) seems redundant, and it makes our API docs look confusing:

Here's my idea: let's create a separate interface for the custom element. We could call it HeadingCustomElementOption or HeadingCustomOption, and it would basically be a copy of HeadingElementOption, but with the model property changed to heading${ string }. You can also try to prepare a basic type and extend it, but I am not sure if it will be correctly compiled by Umberto.

[Mati365]

@DawidKossowski Have you tested this solution? I'm quite sure that it'll lead to collapse of heading union.

[Mati365]

@DawidKossowski I pushed modifications.