[css-forms-1] Add field-sizing to base select styles

Base select is always sized based on its content size rather than using the legacy fixed styling, so we set field-sizing to content !important.

Author: lukewarlow

css-forms-1/Overview.bs

@@ -699,6 +699,11 @@ input[type=radio]:checked::checkmark {
 ## Selects & buttons
 
 ```css
+select {
+  /* Base appearance select always sizes based on its contents. */
+  field-sizing: content !important;
+}
+
 button,
 ::file-selector-button,
 select,