refactor(structure): skip lazy-loading of default pane types

[rexxars]

Edit: This seems to have broken production builds, which now gets

Error: Pane: missing context value

Unclear to me at this time why this happens. Circular imports?

Description

We're currently lazy-loading the different pane types that can appear in structure. The benefit of this is extremely slim, as 99% of all Sanity studios will include both the list, document list and document pane types - and the remaining UserComponent pane is fairly slim.

Additionally, this lazy loading has caused a few code splitting/chunk issues in the past. It also causes an extra delay while loading the module. If we know we're going to need it in most cases, I feel it's better to load it up-front.

This PR removes the lazy loading. I also took the liberty of de-barreling a few of the subfolders here, since we're moving in that direction generally. Also explicitly listed the things that are exported from structure/panes, since I'd prefer us not to blindly export things.

There is a new list in the test studio, "Structure pane types", which can be used to test all the different pane types.

What to review

• Do all the pane types still load as expected? Then we should be good to go 👍
• Are there drawbacks of this refactor that has not been considered?

Testing

I did not add any new tests, as this is baseline functionality that is tested on pretty much every page load.

Notes for release

None. Internal change.