new rule: ARIA required ID references exist

[WilcoFiers]

This PR is related to https://github.com/act-rules/act-rules.github.io/pull/2012, where we took the requirement for ID references to exist in the same tree out. Separating these tests into their own rule allows us to be a little more precise, and as the rules format requires for this to be atomic.

Need for Call for Review: 1 week (because this isn't actually new)

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[MelSumner]

This is great! I like the separation.

[WilcoFiers]

@AndyAtTV Thanks for bringing that up! I completely forgot to check the spec about that. I've changed it to use shadow DOM through a script tag.

[WilcoFiers]

I dismissed all reviews. To rereviewers, I think this is an editorial change so it doesn't need another call for review. Just 3 approvals.

[MelSumner]

Maybe silly question about shadow DOM - should it pass since it still wouldn't be usable for the end user?

I do see the value of checking that it exists, but I wonder about this.

[WilcoFiers]

@MelSumner The shadow DOM example should fail, as it create an accessibility issue. I'm not sure I understood your comment though. Do you want to elaborate?