Allow partial enums

[alvestrand]

I was looking for whether we permit partial enums like we do extended interfaces, and found this bug:

https://www.w3.org/Bugs/Public/show_bug.cgi?id=27048

This doesn't seem to have concluded, but I couldn't find a similar bug in this repo.
FWIW, I think having partial enums is no more dangerous than partial dictionaries or partial interfaces; all of them can lead to name collisions if not well maintained. So I'm (so far) in favor of adding them.

[annevk]

Please don't duplicate issues going forward. Both lists get looked at.

[tobie]

Closing as dup of https://www.w3.org/Bugs/Public/show_bug.cgi?id=27048

[cynthia]

@bzbarsky this looks like it fell into the abyss when Bugzilla was frozen - would it be possible to reopen this?

[bzbarsky]

Yep. I'll check off this bit in #777 too.

[cynthia]

Thank you!

[saschanaz]

I'm seeing some more needs:

• WebXR Augmented Reality Module: People want to add immersive-ar to XRSessionMode from WebXR Device API - Found from Validate IDLs in idl_test() web-platform-tests/wpt#18382
• Clipboard: People want to extend permission types defined by Permissions spec with new clipboard-read and clipboard-write - Found from Add Async Clipboard API permissions microsoft/TypeScript-DOM-lib-generator#735

[annevk]

Even though it's more coordination work, I still think encouraging everyone to maintain centralized lists for these is good, per the rationale in w3ctag/design-principles#99.

[marcoscaceres]

Agree with @annevk. More coordination seems preferable as it provides more checks and balances.