Add .NET Standard 2.0 target

[AArnott]

As discussed in #1415, here is the change to adds .NET Standard 2.0 targeting.

[AArnott]

@thomhurst I got the 3 projects we discussed building. I haven't done anything with the tests to ensure they test the functionality on .NET Framework, which I expect is important. There is also some odd build break that is related to the source generator.

Can you take it from here?

[thomhurst]

Thanks @AArnott ! I'll take a look when I get some time

[thomhurst]

@AArnott looks like I've finally got it working, with the caveat of users need to bring in a poly fill library or add a definition for the ModuleInitializerAttribute to allow the source generated code to register itself

[AArnott]

That is awesome. Your source generator could look to see if the module initializer attribute was already defined in the compilation and if not it could define it itself.

[thomhurst]

That is awesome. Your source generator could look to see if the module initializer attribute was already defined in the compilation and if not it could define it itself.

I was thinking that but if other polyfill libraries are doing the same it'd cause type conflicts. And source generators can't see other source generated code so I wouldn't be able to counter that scenario. So might just have to be a caveat that users define or polyfill it themselves.

[AArnott]

Good point.

[thomhurst]

@AArnott this should now be working in 0.6.0

[AArnott]

I'll try it out on Nerdbank.MessagePack right away. Thanks.

[AArnott]

FYI I was able to get my analyzers test project (which is the smaller of my two test projects) running on TUnit. It required several workarounds however.