fix(structure): use correct loading messages while resolving panes

[rexxars]

Description

While the structure panes are resolving (when using async resolvers), we show one or more loading messages. It starts with a "Loading..." message, shown after X ms. Later, after X seconds, it is supposed to show a "Still loading..." followed by a "Check if there's something wrong with your structure definition" kind of thing.

This code path has been broken for a while now, it seems:

1. The i18n changes had an error, so we're actually showing the resource key instead of the translated message for the first loading message
2. After v3, we checked for a subscribe method on what hopefully is an observable of loading messages, but we returned if that was the case, when we actually wanted the opposite. If what was passed did not have a subscribe method, we would actually have crashed here.

This PR fixes both of these cases.

What to review

• Loading pane shows correct messages (use the workshop story to more easily see it, or create a child structure resolver that waits for 10+s before returning the child node

Testing

I've added a workshop story for this. Ideally I'd also add automated tests, but seems like we're missing some established tests in the structure codebase that I can base them off of.

Notes for release

• Fixed incorrect loading message shown while a pane is resolving in the structure tool

[vercel]

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Name	Status	Preview	Updated (UTC)
performance-studio	✅ Ready (Inspect)	Visit Preview	Mar 20, 2024 4:31pm
test-studio	✅ Ready (Inspect)	Visit Preview	Mar 20, 2024 4:31pm

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[github-actions]

No changes to documentation

[github-actions]

Component Testing Report Updated Mar 20, 2024 4:40 PM (UTC)

File	Status	Duration	Passed	Skipped	Failed
comments/CommentInput.spec.tsx	✅ Passed (Inspect)	32s	15	0	0
formBuilder/ArrayInput.spec.tsx	✅ Passed (Inspect)	6s	3	0	0
formBuilder/inputs/PortableText/Annotations.spec.tsx	✅ Passed (Inspect)	12s	3	0	0
formBuilder/inputs/PortableText/copyPaste/CopyPaste.spec.tsx	✅ Passed (Inspect)	12s	4	2	0
formBuilder/inputs/PortableText/Decorators.spec.tsx	✅ Passed (Inspect)	13s	6	0	0
formBuilder/inputs/PortableText/FocusTracking.spec.tsx	✅ Passed (Inspect)	32s	15	0	0
formBuilder/inputs/PortableText/Input.spec.tsx	✅ Passed (Inspect)	59s	15	0	0
formBuilder/inputs/PortableText/ObjectBlock.spec.tsx	✅ Passed (Inspect)	1m 2s	18	0	0
formBuilder/inputs/PortableText/RangeDecoration.spec.tsx	✅ Passed (Inspect)	12s	6	0	0
formBuilder/inputs/PortableText/Styles.spec.tsx	✅ Passed (Inspect)	13s	6	0	0
formBuilder/inputs/PortableText/Toolbar.spec.tsx	✅ Passed (Inspect)	20s	9	0	0

[binoy14]

makes sense to me, thanks!