Mark `.server.ts` files as being NetworkType server

[Dionysusnu]

If a .server.ts file ends up executing, it will always be as a server script*. This means it will have access to things like ServerStorage, regardless of where the script is located. Therefore, RojoResolver should mark it as having NetworkType server. However, currently, it is always marked according to its location.
For example, a server script under StarterPlayerScripts gets NetworkType client. This generates a false-positive diagnostic when that file imports a ServerStorage file.

*I need to check what rojo uses for precedence between file extension and meta files. Can init.server.ts end up as a LocalScript if the meta.json dictates it?