Editorial: Align with Web IDL specification

[autokagami]

🤖 This is an automated pull request to align the spec with the latest Web IDL specification. 🤖

The followings are the Web IDL validation messages, which may help understanding this PR:

• Validation error at line 4 in periodic-background-sync,2:
    Promise<void> register(DOMString
            ^

  Error: void is now replaced by undefined. Refer to the relevant GitHub issue for more information.

• Validation error at line 4 in periodic-background-sync,2, inside `interface PeriodicSyncManager -> operation register -> argument options`:
  DOMString tag, optional BackgroundSyncOptions options);
                                              ^

  Error: Optional dictionary arguments must have a default value of {}.

• Validation error at line 6 in periodic-background-sync,2:
    Promise<void> unregister(DOMString
            ^

  Error: void is now replaced by undefined. Refer to the relevant GitHub issue for more information.

• Validation error at line 7 in periodic-background-sync,3, inside `interface PeriodicSyncEvent`:
    Constructor(DOMString type,
    ^

  Error: Constructors should now be represented as a constructor() operation on the interface instead of [Constructor] extended attribute. Refer to the WebIDL spec section on constructor operations for more information.

Currently this autofix might introduce awkward code formatting, and feel free to manually fix it whenever it happens.

Please file an issue at https://github.com/saschanaz/webidl-updater/issues/new if you think this is invalid or should be enhanced.