Allow third party to feature its own blocks in the link control panel

[Hug0-Drelon]

What problem does this address?

For the moment, the list of available blocks in the transform section of the link control panel is hardcoded here. It's not possible for a third party plugin to feature a custom block for navigation.

There is still a workaround by adding a random link and transforming it into another kind of block. But from a user point of view it is not convenient at all.

Some context and practical example

Let's take a concrete case with Polylang Pro which provides a language switcher for the navigation block. Unfortunately users can't add it without performing a transform operation on a link block
See bellow what's currently displayed in the transform control panel:

There is no way to easily transform a link into a block other than core/site-logo, core/social-links or core/search.

Instead it would be great to feature our language switcher block like this:

Users will now be able to easily add our block.

Proposed solution

It would be great to open this block list, with a new filter for instance. With such a filter we could easily add blocks.

Maybe linked to this issue #33278

[draganescu]

Wouldn't it be better if the language switcher could be directly (somehow) inserted in the menu, instead of inserting a link first and transforming later?