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

@mgiuca, @dominickng - ok, hopefully N-th time lucky :) 

So, I think we actually do want to throw InvalidStateError if "prompting".  So this would be cover all cases discussed so far (tested with @mgiuca test above too).  

```JS
  prompt() {
    if (this.isTrusted === false) {
      const msg = "Untrusted events can't call prompt().";
      throw new DOMException(msg, "NotAllowedError");
    }

    switch (internalSlots.get(this).promptState) {
      case "done":
        const msg = ".prompt() has expired.";
        throw new DOMException(msg, "InvalidStateError");
      case "prompting":
        const msg = "Already trying to prompt.";
        throw new DOMException(msg, "InvalidStateError");
      default:
        if (this.defaultPrevented === false) {
          const msg = ".prompt() needs to be called after .preventDefault()";
          throw new DOMException(msg, "InvalidStateError");
        }
        internalSlots.get(this).promptState = "prompting";
    }

    (async function task() {
      const promptOutcome = await showInstallPrompt();
      internalSlots.get(this).promptState = "done";
      internalSlots.get(this).userChoiceHandlers.resolve(promptOutcome);
    }.bind(this)())
  }
```

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