fix(fetch-engine): don't use deprecated `url.parse` API
Use `URL` instead of the deprecated `url.parse` API in
`@packages/fetch-engine/src/getProxyAgent.ts` to avoid a scary
warning being printed in the terminal:
```
[DEP0169] DeprecationWarning: `url.parse()` behavior is not standardized and prone to errors that have security implications. Use the WHATWG URL API instead. CVEs are not issued for `url.parse()` vulnerabilities.
```
The new WHATWG URL API has been preferred since Node.js 8.x, and the
legacy `url.parse` API has been formally deprecated since Node.js 11.x.