Add part attribute and note that part and exportparts are superglobal

[wlib]

part and exportparts were the (I hope) only two attributes that I missed in https://github.com/wlib/html-info due to them not appearing in the spec. There was an issue filed in 2018 asking to add part as a superglobal attribute to the DOM spec. This PR also notes that exportparts is a superglobal attribute, but does not make any references to its functionality. I want to ask whether this PR is desirable in the first place because other attributes like those defined in ARIA (e.g. role and aria-label) also are superglobal with corresponding IDL attributes defined on Element. I think that having a table that indexes all of the attributes in one place would be desirable, but it's unclear which spec such an index should live in.

• At least two implementers are interested (and none opposed):
  • …
  • …
• Tests are written and can be reviewed and commented upon at:
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• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (only for aborting and events): …
  • Node.js (only for aborting and events): …
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

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[annevk]

Doing this makes sense to me. However:

1. We need to remove these requirements from the original specification in a PR linked from the commit message.
2. We need to mention these attributes in HTML in the same place we mention id, class, and slot today (can also be a PR linked from the commit message).
3. Do these attributes have no steps to perform when they are manipulated?
4. We should consider, as a follow-up, to introduce a dedicated section on super global attributes to make the concept more formal and also to give some descriptions on the semantics of these attributes as that's currently missing.