Fix TS type reference

[carlpaten]

I'm targeting Node.js 12.x, which means that I shouldn't be able to compile e.g. "foo".replaceAll("a", "b"). But if I import anything from type-fest, from anywhere in my code, then suddenly that code compiles across my entire code base. This is a well-known issue, see e.g. microsoft/TypeScript#33901

The culprit was introduced in #37.

As a partial solution, I would recommend replacing the reference to esnext with a reference to es2020.bigint. Perhaps it would also be worth adding a caveat to the README to the effect that this library is only fully compatible with environments that support es2020.bigint.

[sindresorhus]

Perhaps it would also be worth adding a caveat to the README to the effect that this library is only fully compatible with environments that support es2020.bigint.

That is already implied by the Node.js version target in package.json.