Make any navigation cancel ongoing navigate events

[domenic]

(This is in the broad sense of "navigation", which includes the navigate algorithm, history traversal, and pushState()/replaceState(). I.e., anything that would fire a navigate event, even if it doesn't.)

Discussed in #96 (comment).

Note that this doesn't prohibit calling appHistory.navigate() inside a navigate handler; we want to something more sophisticated for that, with nesting level detection. Instead this ensures that we don't get into a weird situation where

appHistory.addEventListener("navigate", e => {
  if (e.destination === url1) {
    const p2 = appHistory.navigate(url2);
  }
  e.respondWith(Promise.resolve());
});
const p1 = appHistory.navigate(url1);

ends up on url1 because both navigations go through and url2 finishes first. Instead the navigation to url2 goes through and cancels the ongoing navigation to url1, i.e. the second one "wins", which is more what you would expect.

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