Support custom heading-ids and overriding heading id prefix

Is your feature request related to a problem? Please describe.
Currently, heading IDs are prefixed with an h-, implemented here, https://github.com/nextcloud/text/pull/3795, to address naming conflicts reported for headings with Nextcloud, and maybe others.

Rendering does not seem to respect or register custom heading IDs via the {#heading-id} syntax, https://www.markdownguide.org/extended-syntax/#heading-ids.

This way of working around naming conflicts breaks link rendering in my markdown editor when I do local editing (vscodium editor). Because the prefix is hardcoded and custom ids are not respected, I cannot utilize the inter-document links in both editors. I have to chose one or the other, losing functionality either way.

Describe the solution you'd like
Supporting custom heading ids would allow me to set consistent heading links across my editors and allow me to manually address naming conflicts that work with my documents.

Describe alternatives you've considered
Accept the broken links.

Start by reviewing the heading ID implementation linked in the issue and the editor's handling of Markdown heading syntax. Done means {#heading-id} values are respected, the h- prefix can be overridden, and links remain consistent with VSCodium editing.