We should validate the syntax for platform names

[terrajobst]

This related to dotnet/runtime#45851. We should also provide definition-site diagnostics if the syntax of the string passed to the attributes isn't of a valid platform name followed by an optional version.

Note: We don't need to handle the universal methods OperatingSystem.IsPlatform() and OperatingSystem.IsPlatformAtLeast() because the version is a separate argument and dotnet/runtime#45851 will validate that the platform name is known.

[carlossanlop]

@buyaa-n @terrajobst was this approved in an API proposal in the runtime repo? If yes, can you please link the issue number?

[buyaa-n]

@terrajobst sorry, i am not sure what is this issue adding the proposal in dotnet/runtime#45851, i must be missing something. It seems to me this is covered by dotnet/runtime#45851

[buyaa-n]

Solved with #4838