Update list of merged attributes for ...attributes

[rwjblue]

In emberjs/ember.js#17692 (comment) @MelSumner (correctly) called out that there are a number of important attributes other than class that should have the same sort of merging behaviors as class when using ...attributes.

for attributes coming from ARIA, there are five whose value is an ID reference list (meaning, they can have one or more values):

• aria-controls - It seems like the order could possibly matter in some instances.
• aria-describedby: order definitely matters
• aria-flowto - the order could matter, depending on intent of content.
• aria-labelledby - order definitely matters
• aria-owns Order could matter, depending on design intent (if someone is using this, really we should make them stop and think twice tbh, it's smelly),

Rough TODO list:

• Do some research to see if any additional attributes should have these behaviors. We can absolutely do more follow up RFCs so we don't have to be "perfect" here, but a quick review seems prudent.
• We need to author an additional RFC to add these attributes to the list of "merged attributes" which modify the ...attributes behavior.

[rwjblue]

Some background context: emberjs/ember.js#13628

[wagenet]

I'm closing this due to inactivity. This doesn't mean that the idea presented here is invalid, but that, unfortunately, nobody has taken the effort to spearhead it and bring it to completion. Please feel free to advocate for it if you believe that this is still worth pursuing. Thanks!