[w3c/clipboard-apis] Change lastModified to DOMTimeStamp (#138)

[ClipboardItem.lastModified](https://w3c.github.io/clipboard-apis/#dom-clipboarditem-lastmodified) is currently defined as `long long`.

It probably should be `unsigned long long`, but [DOMTimeStamp](https://developer.mozilla.org/en-US/docs/Web/API/DOMTimeStamp) seems like an even better choice.

Is there a good reason to keep this `long long`? Any objections to changing this to `DOMTimeStamp`?

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