Add net461 and net462 targets

[Foxtrek64]

Fixes #9

Blocked by Remora/Remora.Sdk#3 (build will not succeed until this issue is fixed)

[Nihlus]

Is net462 required? The library already builds for netstandard2.0, which claims support for .NET 4.6.2.

[Nihlus]

Also, TupleElementNamesAttribute is still missing with PolySharp installed. I've switched to it in Remora.Sdk anyway because it's more feature-complete, but it seems like this is one piece that's still missing.

[Nihlus]

Follow-up issue: installing System.ValueTuple conditionally fixes that issue, but now I'm getting multiple definitions of IsExternalInit instead.

Related: Sergio0694/PolySharp#48

[Foxtrek64]

Is net462 required? The library already builds for netstandard2.0, which claims support for .NET 4.6.2.

I was looking into this again - Microsoft attempted to retroactively add support for net461 to netstandard2.0, which completely broke and they never actually went and fixed it. Net462 was not affected and will receive support through netstandard2.0.

TL;DR: No, it is not required. Net461 is required because of bugs with the interface between Net461 and Netstandard2.0.

[Nihlus]

I've pushed some experimental changes that don't require PolySharp and it seems to work okay on my end. Can you test and make sure?

[Foxtrek64]

I created a test project in UiPath C# (the more finnicky of the two on this version) with effectively this code:

Result<string> result = "Success!";
MessageBox.Show(result.Entity);

And the message box appeared as expected.

[Nihlus]

I'll close this, then.