Add `{{StringContext}}` extended attribute

[lukewarlow]

Add a {{StringContext}} extended attribute that can be defined on DOMString and USVString.

This is to hook up the Trusted Types validation during the ES->IDL type conversion to avoid issues with its default policy.

See https://github.com/w3c/trusted-types/issues/248, https://github.com/w3c/trusted-types/issues/176

The actual logic of the string validation calls into HTML, and will be something similar to https://w3c.github.io/trusted-types/dist/spec/#html-validate-the-string-in-context.

• [x] At least two implementers are interested (and none opposed):
  • Chromium
  • Gecko
• [ ] Tests are written and can be reviewed and commented upon at:
  • …
• [ ] Implementation bugs are filed:
  • Chromium: Shipping
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1508286 - General trusted types issue
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=266630 - General Trusted Types issue
  • Deno: …
  • Node.js: …
  • webidl2.js: …
  • widlparser: …
• [x] MDN issue is filed: Trusted types itself is documented and the specifics of this IDL attribute probably aren't relevant.
• [ ] The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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Preview | Diff

[lukewarlow]

Fork of #841 will address review comments here

[yuki3]

I think this looks reasonable, although I think we probably should note the overload limitations more explicitly.

@EdgarChen @petervanderbeken @yuki3 any of you want to take a look as well?

Looks good to me, too.

[yuki3]

cc @natechapin as he's taking over the Blink-V8 bindings work (including interop work) from me.

[petervanderbeken]

I do think that we need to make [StringContext]-annotated strings not distinguishable from object, interface-like, callback function, dictionary-like and sequence-like. So we probably need to add another letter in the table, and use it in the string types row to explain that these are only distinguishable if the string type doesn't have a [StringContext] extended attribute.

[lukewarlow]

Based on discussions here, in Matrix and a video call we had with Koto and Daniel I'm going to go ahead and close this PR. If Daniel discovers a reason why the observable timing differences between IDL and call site aren't web compatible we can revisit this.