[w3c/clipboard-apis] Specify how to handle unsupported data types in `write()`. (#128)

I think there are a few reasonable options:

- Return which data types were copied, e.g. give `write()` a return type of `Promise<Array<{[type: string]: boolean}>>`.
- Specify that the browser should error.
- Specify that the browser should ignore it.

It would also be useful if there was a way to query for supported types (#67), especially in the last case.

----------

Consider the following snippet:

    document.body.addEventListener("click", async function() {
      console.log("copying!");
      const item = new ClipboardItem({
        "food/falafel": new Blob(["\uD83E\uDDC6"], { type: "food/falafel" })
      });
      await navigator.clipboard.write([item]);
      console.log("copied!");
    });

Chrome throws an error:
<img width="490" alt="Screen Shot 2020-06-26 at 16 05 17" src="https://user-images.githubusercontent.com/248078/85907585-d731e380-b7c6-11ea-8953-82846246ab11.png">

Safari resolves the Promise without an error.
<img width="534" alt="Screen Shot 2020-06-26 at 16 05 57" src="https://user-images.githubusercontent.com/248078/85907628-ef096780-b7c6-11ea-8b6b-ac59478705b9.png">

Note that since the return type is `Promise<void>`, there is no other way (in general) to check that the copy operation succeeded.

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