Move to ESM and require minimum node 16 (#32)

* Refactor: move to [ESM](https://nodejs.org/api/esm.html), refer to https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c
* Update doc and comments.
* Require minimum node version 16.

README.md

@@ -16,8 +16,7 @@ npm i @xn-02f/md5
 ## Usage
 
 ```javascript
-const md5 = require('@xn-02f/md5');
-// import md5 from '@xn-02f/md5'
+import md5 from '@xn-02f/md5'
 
 md5('xn-02f');  // => '54d30fa674d13e3598970bc9c5e2388e'
 ```

md5.d.ts

@@ -5,11 +5,10 @@
  * @return {string} The hexadecimal hash string that is handled and converted.
  * @example
  * ```
- * import md5 = require('@xn-02f/md5');
- * // import md5 from '@xn-02f/md5';
+ * import md5 from '@xn-02f/md5';
  * md5('xn-02f'); //=> '54d30fa674d13e3598970bc9c5e2388e'
  * ```
  */
 declare const md5: (data: string) => string;
 
-export = md5;
+export default md5;

md5.js

@@ -11,12 +11,11 @@
  * @return {string} The hexadecimal hash string that is handled and converted.
  * @example
  * ```
- * const md5 = require('@xn-02f/md5');
- * // import md5 from '@xn-02f/md5';
+ * import md5 from '@xn-02f/md5';
  * md5('xn-02f'); //=> '54d30fa674d13e3598970bc9c5e2388e'
  * ```
  */
-module.exports = data => {
+export default data => {
     const stringUTF8 = unescape(encodeURIComponent(data));
 
     const stringNumArr = rstr2binl(stringUTF8);

package.json

@@ -2,6 +2,8 @@
   "name": "@xn-02f/md5",
   "version": "2.4.4",
   "description": "A tiny MD5 library without dependencies for node, convert string to MD5 hash.",
+  "type": "module",
+  "exports": "./md5.js",
   "main": "md5.js",
   "types": "md5.d.ts",
   "scripts": {
@@ -24,6 +26,9 @@
     "ava": "5.x",
     "tsd": "^0.28.1"
   },
+  "engines": {
+    "node": ">=16"
+  },
   "files": [
     "md5.js",
     "md5.d.ts"

test/md5.test.js

@@ -1,9 +1,9 @@
 /*
  * This file is used to test.
  */
-const test = require('ava');
+import test from 'ava';
 
-const md5 = require('../md5');
+import md5 from '../md5.js';
 
 // Instance object for testing
 const example = {