[css-view-transitions-2] 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 9 in css-view-transitions-2,2, inside `interface Document -> operation startViewTransition -> argument callbackOptions`:
   (UpdateCallback or StartViewTransitionOptions) callbackOptions);
                                                  ^
  

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

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.

[saschanaz]

Thanks bot for trying so hard to make this merged 😄 (and thanks for finally merging it!)