[AriaNotify] Naming for the priority property

[alisonmaher]

The current design of AriaNotify, as outlined in the spec PR and the explainer, includes a priority, which takes on values of none or important.

These map closely to the aria-live values of polite and assertive. Do we want the naming to closely align with these, or do we like that this diverges to help authors in better distinguishing these two features?

My proposal is to keep the naming separate given that polite and assertive may imply certain assumptions surrounding interrupt, which is a separate property in the ariaNotify API.