Heading anchors / "copy link to heading" as an opt-in extension

[ihordubas99]

Context
This was discussed in #1180 and closed as application specific; a related request for heading id attributes was also closed (<link to Ovgodd's discussion>), and in-document #hash links were requested in <link to ssa-serg's discussion>. We understand the concerns raised there (URL handling, multiple editors per page, UI). We'd like to propose a narrower, opt-in version, since several integrators (OpenProject, La Suite Docs – see #2003, suitenumerique/docs#653) are building the same thing independently.

What we built in OpenProject
Headings in collaborative documents get a link button. Clicking it copies a link to the heading, and opening the link scrolls to it.

• The anchor is the block id (#heading-<id>), so links survive heading renames and nothing extra is stored in the document.
• The extension scrolls to the target itself (it also works when the editor lives in a shadow root and content arrives after page load), opens collapsed toggles around the target, and keeps it in view while content above is still loading.
• Decorations are only rebuilt when an edit can affect a heading, so typing in long documents stays cheap.

Proposal
An opt-in extension (off by default) that:

• renders a link affordance on heading blocks,
• exposes a hook/callback to build the URL and to handle the copy, so the application stays in control of URL format and of what happens with multiple editors on a page,
• provides a scrollToBlock(id) helper that integrators can call on load / on hashchange.