Implement IL2C specialized tfm

[kekyo]

Implement IL2C specialized tfm

[kekyo]

The motivation for the stub assembly came from #91 . I was hesitant at first because of the seemingly high cost of maintaining this, but I am inclined to say that it might be better to maintain it now (before we are plagued with a large number of APIs later). As a first version, manual maintenance might be fine (provided it is in a format and method that can be automated later).

[kekyo]

Bit related #52

[kekyo]

I found it from NuGet.Frameworks repo: https://github.com/NuGet/NuGet.Client/blob/f1c5b47018d84ff0bff7aa0fc5fedc7c3db95f34/src/NuGet.Core/NuGet.Frameworks/DefaultFrameworkMappings.cs#L10

It implements IFrameworkMappings to provide a specialized key dictionary of the default set of tfm components. As for the NuGet.Frameworks library, an interface consisting of this interface and IFrameworkNameProvider is available as an extension point.

However, this default key dictionary is still hard-coded, so it appears that new keys cannot be inserted without recompiling the library for calls from existing code in MSBuild 💀