Previously, we were guessing whether something is optional from the
comment (looking for stuff like "defaults to" or "optional").
To remove this error-prone technique, every optional property or
argument (but not option) is explicitly marked with "?".
This makes dialogs disappear and prevents stalling.
Pros:
- No need to worry about dialogs for most users.
- Those that wait for a specific dialog still get to control it.
Cons:
- Those who use Playwright to show interactive browser will have
to add an empty 'dialog' handler to prevent auto-dismiss.
We do this in cli.
