fix: (useCopyToClipboard): copyToClipboard should return promise

[kendrickw]

Fixes #312

The copyToClipboard function is not returning a promise currently. The intention (at least according to the function signature) is to fulfill the promise when the text is copied:

usehooks/index.d.ts

Lines 126 to 129 in 90fbbb4

	export function useCopyToClipboard(): [
	string | null,
	(value: string) => Promise<void>
	];

This PR will change the code to return a promise. So the promise can fulfill when the text is copied successfully.