Escape trailing spaces in opaque paths

[karwa]

Resolves #784

I'll do the tests/bugs/etc if we're okay with this approach (escaping trailing spaces rather than trimming them). PR is for discussion at this point.

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• [ ] At least two implementers are interested (and none opposed):
  • …
  • …
• [ ] Tests are written and can be reviewed and commented upon at:
  • …
• [ ] Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno: …
  • Node.js: …
• [ ] MDN issue is filed: …

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

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