Update polyfill to align with v1 implementations

[smockle]

Decisions reached in w3c/aria#2426 and w3c/aria#2333 require two polyfill changes:

• Remove interrupt.
• Change priority’s values to "high" and "normal".

[smockle]

cc @keithamus, in case you have thoughts re: keeping interrupt in a separate branch (future perhaps, to align with the explainer’s “Future considerations”?) versus deleting it.

[keithamus]

We always have the git history, so I am not so concerned about keeping it on main.

[smockle]

Opened #10 to implement this.