Expose AbortError

[timdp]

Currently, if I pass in an AbortSignal and want to detect why the promise rejected, I have to write:

import fetch from 'minipass-fetch'
import AbortError from 'minipass-fetch/lib/abort-error'

... in order to use the type:

try {
  await fetch('https://example.com', { signal: AbortSignal.timeout(1000) })
} catch (err) {
  if (err instanceof AbortError) {
    // Timed out
  } else {
    // Something else went wrong
  }
}

This PR turns the preamble into:

import fetch, { AbortError } from 'minipass-fetch'

... which is in line with how we're already exporting FetchError.

---

Playing devil's advocate:

• I could also check err.name, err.code, or err.type, but none of those are guaranteed to originate from minipass-fetch. I'm not a fan of duck typing when it comes to errors.
• Other runtimes don't support this scenario either:
  • Node 17.7.2 (with --experimental-fetch) generates a vanilla Error with code = 'ABORT_ERR'.
  • Chrome generates a DOMException.
  • I didn't check Firefox, Safari, Deno, or Cloudflare Workers.

[nlf]

we expose FetchError already so following that precedent for AbortError totally makes sense to me. thanks for the contribution!