Align .editorconfig with .stylelintrc for CSS files (#3912)

And add explicit file extension to `.stylelintrc`

.editorconfig

@@ -8,6 +8,9 @@ end_of_line = lf
 insert_final_newline = true
 trim_trailing_whitespace = true
 
+[*.{css,scss}]
+indent_style = tab
+
 [*.{html,php,phtml}]
 indent_size = 4
 indent_style = tab