Re: [w3c/manifest] Add installation prompt control flow (#417)

I need to give this more thought... but my initial reaction is that calling prompt() multiple times or firing multiple events doesn't feel right.

Rough sketch here, if we were to keep backwards compat with current Chrome implementation - but actually just give developers what they are asking for:

```JS
async () => {
  let userChoice;
  // We add navigator.canPromptForInstall
  // that waits for installability signal
  // Resolves to true or false, because end-user may tell UA "don't bug me with installs, ever!" 
  if (await navigator.canPromptForInstall) {
    // BIP compat layer - except, browser no longer fire this on their own!
    window.beforeintallprompt = ev => {
      if (thingsThatBlockInstall.length === 0) {
        return;
      }
      ev.preventDefault();
      await Promise.all(thingsThatBlockInstall);
      ev.prompt(); // show it... it settles "navigator.requestInstallPrompt()"
    }
    userChoice = await navigator.requestInstallPrompt();
  }
}();
```


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