LinkControl: Add support for linking headings

[richtabor]

When applying an inline link, it'd be nice to be able to link to headings that exist within the post content.

The idea here is that all available headings would be available to select. When you select one, an anchor tag is automatically applied to the corresponding heading, as the anchor link is applied to the selected text.

This UX leverages the existing pattern of having additional tools within the footer of the link popover, as discussed in #50891. The pattern is also explored in this proposal (#50888) for adding blocks within the navigation link block. We should build this out pattern in a way where it could be shared across both.

Visual

cc: @getdave, as you've worked extensively on LinkControl. 🤜🤛

[richtabor]

Noting that this should only be available when linking inline — and if there are headings available.

[getdave]

Noting that if we want to achieve this we need to create the "back" functionality between Popovers. It would need to be configureable. This is not something we currently support.

The alternative would be to add this component as part of LinkControl, but we should first consider whether we can instead use some kind of extension point to avoid bloating that component.

[richtabor]

Noting that if we want to achieve this we need to create the "back" functionality between Popovers.

Perhaps? It's essentially a state of the popover contents. Add heading link could change the state, back could always revert to the initial. Seems it may be a heavy lift to get it in popover though, if there's no other use cases outside of LinkControl (at least currently).

There're two related to LinkControl, this and #50888.