Adjust editor config to reflect cast conventions in C# libraries

Most of our code includes a space after the cast, e.g. `(int) x`
instead of `(int)x`. This .editorconfig change means that will be
the default formatting when it's performed by the editor.

Author: jskeet

csharp/.editorconfig

@@ -14,4 +14,7 @@ tab_width = 4
 # New line preferences
 end_of_line = lf
 insert_final_newline = false
-trim_trailing_whitespace = true
+trim_trailing_whitespace = true
+
+[*.cs]
+csharp_space_after_cast = true