Switch trusted types sinks to use union type

[lukewarlow]

Switch Trusted Types sinks to use union IDL type

After recent discussions we've decided to change trusted types to use union IDL types across all sinks. This requires changes to the IDL and algorithms for the various sinks.

• At least two implementers are interested (and none opposed):
  • Gecko Trusted Types mozilla/standards-positions#20
  • Chromium
• Tests are written and can be reviewed and commented upon at:
  • https://wpt.live/trusted-types
  • WebKit export of https://bugs.webkit.org/show_bug.cgi?id=273412 web-platform-tests/wpt#46185
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/340822358
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1508286 - meta bug
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=273412
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

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

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[zcorpan]

The diff view shows document.write and writeln with HTMLString. I suppose this needs to be rebased so that #10199 is included, and update those to union type here.

[lukewarlow]

I've rebased atop the document.write PR and removed the strings from this PR

[lukewarlow]

@zcorpan @annevk is this ready to be merged now it's rebased?