Add a BoundaryPointsRange mixin

[annevk]

This allows AbstractRange to be empty so it can be the superclass of OpaqueRange (which won't expose its boundary points).

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[stephanieyzhang]

lgtm - thanks for creating this! I'll rebase #1404 on top once this lands.

fwiw, I grepped the WHATWG HTML source and csswg-drafts for {{AbstractRange/...}} refs that might break due to this rename and didn't find anything, but flagging just in case I missed something.

[annevk]

Oh good call. Per https://dontcallmedom.github.io/webdex/a.html#AbstractRange%40%40%40%40interface and in particular https://dontcallmedom.github.io/webdex/c.html#collapsed@@AbstractRange@attribute this would break https://w3c.github.io/editing/docs/execCommand/#effectively-contained. Although arguably that is in violation of the rule that you cannot define an algorithm in terms of public API. cc @zcorpan

I can create a patch for that tomorrow. I think the observable differences will be covered by the generated IDL tests, but I should also mention that in the eventual commit message for clarity.

[annevk]

#1470