Request for new `webdriver-event` page type

For WebDriver, we already have these [page types](https://developer.mozilla.org/en-US/docs/MDN/Writing_guidelines/Page_structures/Page_types/Page_type_key#webdriver_page_types):

- `webdriver-command`
- `webdriver-capability`
- `webdriver-error`

Since BiDi brings events to WebDriver, we'll need a new page type for `webdriver-event`.