Support for async dynamic attributes

[radekmie]

As I wrote in #428, async dynamic attributes (i.e., <img {{getAttributesAsync}}>) is not supported yet. I believe it should be enough to cover it with something like waitForAllAttributesAndContinue for standard attributes, but also lifted for arrays (i.e., support arrays of objects with Promises).

Currently, it can be worked around by using an additional #let.

[jankapunkt]

@radekmie can you shortly explain, why it's not working? Is it the way how the updater is wrapped in the autorun or because the flatten and expand functions are all sync or even something else?

I was playing around a bit and Promises resolve without breaking tests, but the attributes were in the end not set. I'd like to understand why that is

[radekmie]

No, there are no "limitations". It's just a different part of code (not shared with singular attributes) and it'd require separate implementation. I have it partly working locally and plan to finish it by the end of the next week.

[jankapunkt]

@radekmie you can also create a draft PR and I can chime in if that makes sense