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

To be clear - I think this is what we want:

```JS
async function tryToInstall() {
  if (!("canPromptForInstall" in navigator) || !(await navigator.canPromptForInstall)) {
    return; // Computer says no. 
  }
  await Promise.all(thingsThatBlockInstall);
  try {
    const userChoice = await navigator.requestInstallPrompt();
    doWhateverWith(userChoice);
  } catch (err) {
    // catch in case we missed our window of opportunity,
    // or we called it too soon after page load,
    // or we called it without user interaction,
    // or we got rate-limited,
    // or an invalid state because user is doing it manually now,
    // or whatever...
  }
}
installButton.onclick = tryToInstall;
```


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