Move C# mappings to configurable directory

[iamcarbon]

Syntax...

{
   types = {
      "String"    = "System.Text.Utf8String"
      "Float32" = "float"
      "Float64" = "double"
       "X" = "x,Assembly@Version"
   }
}

Notes: Assume .NETSTANDARD 2.0 is imported.