fix insertBetween typings

src/lib/array.ts

@@ -10,7 +10,7 @@ export function splitBy(fn) {
 }
 
 export function insertBetween<T, I = T>(items: T[], fn: (i: number) => I): Array<T | I> {
-    return items.reduce((acc, x, i) => {
+    return items.reduce<Array<T | I>>((acc, x, i) => {
         acc.push(x)
 
         if (i < items.length - 1) {