Implement number average util

.changeset/silver-cheetahs-jam.md

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+---
+'@busyxiang/utilities-js': patch
+---
+
+Add number average util

src/number.test.ts

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+import { average } from './number';
+
+describe('average', () => {
+  test('it should return average value', () => {
+    const aver = average(1, 2, 3);
+
+    expect(aver).toBe(2);
+  });
+
+  test('it should return average value(spread array)', () => {
+    const aver = average(...[1, 2, 3]);
+
+    expect(aver).toBe(2);
+  });
+});

src/number.ts

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+export const average = (...nums: number[]) => {
+  return nums.reduce((acc, value) => acc + value, 0) / nums.length;
+};