Add experimental debug-only file association for Multiplayer saves

[mibac138]

Introduces a .rwmts extension for the assocation. Currently unused by Multiplayer itself - only created manually.

[notfood]

I can't test this under Linux. Won't having Microsoft.Win32 cause issues for other OSes? I've skimmed through some random reports on the internet.

[mibac138]

From my understanding just referencing Microsoft.Win32 isn't enough to cause issues by itself. (see here)

Platform-specific APIs will fail on other platforms. For example, Microsoft.Win32.Registry will succeed on Windows and throw PlatformNotSupportedException when used on any other OS.

Because of that, all access to the FileAssoc class is guarded by the platform == Windows check, so this shouldn't cause issues on other platforms - the functionality just isn't available. I've also added the platform check to the methods of FileAssoc to guard against accidental misuse.