Export default diagnostics extractors

[DaniPopes]

The config diagnostic_extractors provided by the rustc feature are not exported but reachable through Config::{cargo,rustc}().diagnostic_extractor; making these public would let users import the function directly instead of having it to access it through Config methods.
I've renamed them and moved them to the diagnostics module, as well as added the default one as a separate function.

My use case for this is I am testing another compiler that can emit JSON diagnostics using the rustc format, but everything else about the configuration will be different, so it doesn't really make sense to call Config::rustc if it wasn't for this method.

BTW I was able to migrate to this library from a compiletest-rs fork pretty easily, thanks a lot for developing this!

[oli-obk]

Good idea. It should be possible to build a rustc config manually without the rustc constructor function. There's still more I need to expose, but this is an important piece