Consistently use Promise<void> instead of Promise<undefined>

[lucacasonato]

lib.dom.d.ts ubiquitously uses Promise<void> for promises not resolving to undefined. It has done this for a long time.

When the generator was written, there were 0 or 1 (I can't quite tell) places where this type of promise occured outside of a function return value. However, Web Streams are now widely supported, and they have properties that are promises that resolve to no value. These are currently typed as Promise<undefined>. This commit changes them to be typed as Promise<void>, to align with the rest of lib.dom.d.ts.

There are 5 readonly properties that are impacted by this change.