[TS] Fallback and requires properties on elements

[andrewleader]

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TS implementation of #2019

[andrewleader]

@dclaux looks like this is implemented in the renderer, but the designer doesn't have support for specifying fallbacks (or requires). How do you want to track designer work for new features?

We could...

1. Create two issues, one for the TS implementation, and one for the Designer implementation, and therefore they're both tracked independently
2. Use the single TS issue for tracking both the TS renderer implementation and the Designer implementation (meaning that the TS issue will be blocked from being closed till the Designer work is finished)

What approach would you like? If the Designer implementation often comes much later, then it might make sense to split them apart... however I think you usually get it done around the same time... so let me know which approach works best for you!

[andrewleader]

Closing since TS implementation is complete, we'll have a separate task to track Designer work.