IPv6 support

[OlliV]

Currently we don't try to resolve to an AAAA record at all.

1. Check if IPv6 connectivity is available (either actively or passively by inspecting the configuration)
   a. IPv6 avail: Resolve both AAAA and A records concurrently
   b. IPv4-only: Resolve A record only
2. Create possible URLs for both IPv6 and IPv4
3. Fallback to IPv4 on a network error

https://github.com/OlliV/fetch/blob/5853c7bc4a94504409d0a7e2d70b3935752ecfce/src/index.ts#L82