[BUG] Typescript typings don't work

## 🐛 Bug Report

Looks like typescript d.ts file is wrong: typings are not suggested at all (using webstorm).

Lib version: 4.3.0

## Steps To Reproduce

```typescript
import { Workbook } from 'exceljs/index';
const workbook = new Workbook();
const worksheet = workbook.addWorksheet(name); // <--- unresolved function method "addWorksheet()"
```

It looks like *.ts file imports js implementation as "*" in index.ts file which is tricky and not well supported.

What should be done:
Simply rename index.ts -> index.js (since "ts" makes no sense in the way it is used)