[css-forms-1] Add UA style to give slider fill an inline size of 0 when control is indeterminate. (#12428)

Author: lukewarlow

css-forms-1/Overview.bs

@@ -825,6 +825,10 @@ ISSUE: Refine meter, progress, switch and range input styling.
   background-color: currentColor;
 }
 
+:indeterminate::slider-fill {
+  inline-size: 0;
+}
+
 ::slider-thumb {
   border-radius: 0;
   border: none;