fix: make mixins declaration backward compatible (#436)

Author: ktsn

src/util.ts

@@ -46,6 +46,13 @@ export type MixedVueClass<
   ? VueClass<UnionToIntersection<ExtractInstance<T>>>
   : never
 
+// Retain legacy declaration for backward compatibility
+export function mixins <A> (CtorA: VueClass<A>): VueClass<A>
+export function mixins <A, B> (CtorA: VueClass<A>, CtorB: VueClass<B>): VueClass<A & B>
+export function mixins <A, B, C> (CtorA: VueClass<A>, CtorB: VueClass<B>, CtorC: VueClass<C>): VueClass<A & B & C>
+export function mixins <A, B, C, D> (CtorA: VueClass<A>, CtorB: VueClass<B>, CtorC: VueClass<C>, CtorD: VueClass<D>): VueClass<A & B & C & D>
+export function mixins <A, B, C, D, E> (CtorA: VueClass<A>, CtorB: VueClass<B>, CtorC: VueClass<C>, CtorD: VueClass<D>, CtorE: VueClass<E>): VueClass<A & B & C & D & E>
+
 export function mixins<T extends VueClass<Vue>[]>(...Ctors: T): MixedVueClass<T>
 export function mixins (...Ctors: VueClass<Vue>[]): VueClass<Vue> {
   return Vue.extend({ mixins: Ctors })