feat: remove default export, ratelimit must now be imported directly

BREAKING CHANGE: default export is removed, import the ratelimit function directly
```ts
// before
const ratelimit = require('koa-simple-ratelimit');
import ratelimit from 'koa-simple-ratelimit';

// after
const { ratelimit } = require('koa-simple-ratelimit');
import { ratelimit } from 'koa-simple-ratelimit';
```

README.md

@@ -25,7 +25,7 @@ npm install koa-simple-ratelimit
 import Koa from 'koa';
 import redis from 'redis';
 
-import ratelimit from './src';
+import { ratelimit } from 'koa-simple-ratelimit';
 
 const app = new Koa();