fix: missing return type for del (#49)

Author: zigomir

lib/index.d.ts

@@ -4,7 +4,7 @@ declare const isVue3: boolean
 declare const install: (vue?: any) => void
 
 export function set<T>(target: any, key: any, val: T): T
-export function del(target: any, key: any)
+export function del(target: any, key: any): void
 
 export * from 'vue'
 export {