Support NET Standard 2.0

[hotchkj]

Older tooling does not appear to load the existing platform targets (for example if I try and load Microsoft.DiaSymReader.Converter into Cake 1.0 it indicates 'no matching framework'), and if I clone the code and its dependencies, they all appear to compile to NET Standard 2.0. This would be useful for the intermediate timescale before everyone can move to NET 5.