Add trim_trailing_whitespace to editorconfig

.editorconfig

@@ -21,6 +21,7 @@ csharp_new_line_before_finally = true
 csharp_new_line_before_members_in_object_initializers = true
 csharp_new_line_before_members_in_anonymous_types = true
 csharp_new_line_between_query_expression_clauses = true
+trim_trailing_whitespace = true
 
 # Indentation preferences
 csharp_indent_block_contents = true