Form Block: prioritize inner blocks in the inserter

[jeherve]

When using the Form block, once you've inserted a basic form you can add additional blocks inside the form. Most people will use that option to add additional fields, such as checkboxes, radio boxes, or text fields they may need in their form in addition to the default ones.

Today, when you want to add an additional block, core blocks come up first:

Ideally, we would show Jetpack Form inner blocks in there first.

This should now be possible starting in WordPress 6.3, using the new prioritizedInserterBlocks API introduced in WordPress/gutenberg#50510
Let's see if we can leverage that in Jetpack.

Primary issue: #31638