change the global `Clipboard` to a class
DOM builtins like `Clipboard` have values at runtime, because you can do
e.g. `navigator.clipboard instanceof Clipboard`.  This is better modeled
by a class.
Relevant section of the spec is
where the interface is tagged as `Exposed=Window`.
*/
/**
- * @interface
- * @see https://w3c.github.io/clipboard-apis/#async-clipboard-api
+ * @constructor
+ * @see https://w3c.github.io/clipboard-apis/#clipboard-interface
*/
function Clipboard() {}
