Use .ToLowerInvariant, which is supported by coreclr

I'd argue that at this stage, this should probably be the default. Further options to consider:

- Make it a singleton, or at least expose a static property with a single instance?
- Allow SetLogger(null) to use the NullLogger?

Coreclr doesn't support Assembly.GetExecutingAssembly().
Use TypeInfo.Assembly instead, which is supported on all platform.

Coreclr uses different platform detection primitives to full .NET