Cursor support for FileSystemSyncAccessHandle

[jesup]

Adds cursor to FileSystemSyncAccessHandle, similar to the cursor for WritableFileStream. This means that supplying an 'at:' parameter for every read and write will not be needed.

• [ ] At least two implementers are interested (and none opposed):
  • Mozilla
  • Google
• [ ] Tests are written and can be reviewed and commented upon at:
  • https://phabricator.services.mozilla.com/D163399
• [ ] Implementation bugs are filed:
  • Chromium: https://crbug.com/1394790
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1803241
  • WebKit: …
• [ ] MDN issue is filed: …

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

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[jesup]

Thanks for the careful review, @annevk !

[jesup]

Note that the existing cursor for WritableFileStream should have normative text for [=FileSystemWritableFileStream/file position cursor=]; right now all references are in non-normative text. I'm considering this a separate issue

[a-sully]

Question: with this change, should FileSystemReadWriteOptions.at become optional rather than defaulting to 0? Otherwise the behavior for read and write calls using default options is different than if you call the version of the method that does not specify any options

[annevk]

Filed #81.