Create type definition for 'deepcopy' (#4000)

src/@types/deepcopy.d.ts

@@ -0,0 +1,17 @@
+declare module 'deepcopy';
+
+declare namespace deepcopy {
+	type DeepcopyCustomizerValueType = 'Object';
+
+	type DeepcopyCustomizer<T> = (
+		value: T,
+		valueType: DeepcopyCustomizerValueType) => T;
+
+	interface DeepcopyOptions<T> {
+		customizer: DeepcopyCustomizer<T>
+	}
+
+	export function deepcopy<T>(
+		value: T,
+		options?: DeepcopyOptions<T> | DeepcopyCustomizer<T>): T;
+}