Lots of editorial spec bugs here, but these changes largely affect how
the unhandledPromptBehavior capability is handled. We also now set an
additional capability for the default User Agent string.
WebDriver script authors may now provide either:
* A user prompt handler configuration to be used for all prompt types.
* A set of per-prompt-type user prompt handlers.
This also paves the way for interaction with the beforeunload prompt,
though we do not yet support that feature in LibWeb.
See: 43903d0
