.editorconfig: trim trailing whitespaces

I think we don't want trailing whitespaces anywhere in this repository

.editorconfig

@@ -4,6 +4,7 @@ root = true
 end_of_line = lf
 charset = utf-8
 insert_final_newline = true
+trim_trailing_whitespace = true
 
 [**.json]
 indent_style = space