feat: drop custom Unknown type

`unknown` is now a built-in keyword in typescript. Since we will no
longer be supporting pre-3.0 versions of typescript, we can drop the
`Unknown` helper.

BREAKING CHANGE: `Unknown` is no longer supported. Can find and replace
all instances of `Unknown` with `unknown` to maintain same behavior.

src/types/utils.ts

@@ -14,11 +14,6 @@ export type NoInfer<T> = T & ObjectType<T>;
  * @returns `T | null | undefined`
  */
 export type Nullable<T> = T | null | undefined;
-/**
- * A constant type that mimics an unknowable type.
- * @returns `{} | null | undefined`
- */
-export type Unknown = {} | null | undefined;
 
 /**
  * no-doc - This is a helper for `Nominal` and is not useful on its own