Add new IL2C.Toolchain.msvc

[kekyo]

#79 totally omitted IL2C.Runtime.msvc NuGet package.
Have to add new driver for MSVC by IL2C.Toolchain.msvc same interface as IL2C.Toolchain.gcc4.mingw32.

This is because it cannot contain VC binaries :

• Detect installed VC
• reflect its configuration in the PropertyGroup

and it should be a small package.

Maybe it could be written in an inline task in MSBuild (inline tasks have strong restrictions on reference assemblies, so if we can't use Microsoft.Win32.Registry, we need to collect the information in an external process or write a custom task). Custom tasks have platform compatibility issues, so if we do it, it will be an external process.

(Even in #79, the custom task is replaced with an external process drive.)

[kekyo]

The current compiler driver implementation has some hard-coding that is dependent on gcc in places.
To completely eliminate this problem, it is thought that a kind of template engine or string replacement is needed for command line generation of compiler execution.

• ex: Archiver execution option is hardcoded rcs.

I feel this topic is a little big to frame for VC support, might split the issue.