Allow saving component compositions as sections (frontend only)

This is not meant for #3454094: Milestone 0.1.0: Experience Builder Demo nor for #3455753: Milestone 0.2.0: Early preview, right?

Not actionable yet, so removing from https://contribkanban.com/board/experience_builder.

This is part of the #3455753: Milestone 0.2.0: Early preview scope.

#7: 🥳

FYI, the UI pieces are blocked on design (yay for @bostonjillian working on this!), but that doesn't mean the back-end pieces cannot already be built! Missing config entity infrastructure has a precise definition of what is needed at #3479643: Introduce a `Pattern` config entity, and #3479982: HTTP API to read+write PageTemplate and Pattern config entities will make it possible to let the client list/read/create/edit/delete "component compositions" aka "sections" aka "patterns" 👍

So this is postponed on 3 distinct things ⇒ prefixing title with [PP-3].

But note that only the design part is a hard blocker.

Yay!

Is it true that deleting a section would remove it wherever it’s used? I guess it makes sense.

Is there any plans for archiving a section so it’s still shown where previously added but not available for adding to new things? Or perhaps tagging and hiding as part of the categorization feature that’s being designed?

@kristen pol that screenshot certainly appears to be showing that, however I am not sure it's a good idea or technically necessary. I have a design sync call later today and I'll seek clarification on this.

Good news 🙏

I'm back! Thank you @kristenpol and @jessebaker for your feedback, especially the clarification on the delete behavior.
Lauri and I met to review, iterate, and make design decisions on sections. Here's a quick overview of where we landed:
10/29 update: Designs have been updated to better reflect section behavior. Sections use groups of non-synced elements + components that have been saved for reuse in the library. When initially creating a section, a modal appears to name that section in both the library and on the canvas. That section can be renamed in the library; however, it will not be reflected on the canvas as they are not synced. Similarly, deleting a section from the library does not affect any instance of that section used on the canvas.

I also added a link to the Figma file if anyone is interested in getting a closer look.

#3479982: HTTP API to read+write PageTemplate and Pattern config entities landed, so that reduces the postponedness.

I'm leaving [PP-2] in the title, however, I was able to make progress on the front end side of this ticket and have raised an MR that can be merged even though ultimately clicking "Save to library" won't actually work yet - it does allow you to get all the way to that pont!

#3479982: HTTP API to read+write PageTemplate and Pattern config entities is in, we're now just waiting for #3479982: HTTP API to read+write PageTemplate and Pattern config entities!

Looks good to me, just one question about LayoutNode type vs. RootNode type.

Thanks all!

Created follow-up to integrate the frontend with the backend, after the backend patterns work is done:
#3486203: Integrate saving sections with the backend: (de)normalize Pattern config entities using the client-side data model

Renaming title to emphasize it is frontend only

#3486203: Integrate saving sections with the backend: (de)normalize Pattern config entities using the client-side data model is now unblocked!