message: doc(readme): Split download badge
diff: <p align="center">
  <a href="https://www.npmjs.com/package/@swc/core">
  - <img alt="npm Downloads" src="https://img.shields.io/npm/dm/@swc/helpers">
  + <img alt="downloads (@swc/core)" src="https://img.shields.io/npm/dm/@swc/core?label=downloads%20%28%40swc%2Fcore%29">
  </a>
  + <a href="https://www.npmjs.com/package/@swc/helpers">
  + <img alt="downloads (3rd party)" src="https://img.shields.io/npm/dm/@swc/helpers?label=downloads%20%283rd%20party%29">
  + </a>
  +</p>
  +<p align="center">
  <a href="https://crates.io/crates/swc_ecma_parser">
  <img alt="undefined" src="https://img.shields.io/crates/d/swc_ecma_parser.svg?label=crates.io%20downloads">
  </a>
  