Add CSS Utility for auto width and height Properties (#24249)

Currently bootstrap only provides 4 variations of `width` and `height` css utilities, which are:

1. 25%
2. 50%
3. 75%
4. 100%.

Here, I'm adding `auto` to the list. This is useful when we need to reset the width/height value
back to the browser default value at some point.

scss/_variables.scss

@@ -132,7 +132,8 @@ $sizes: map-merge((
   25: 25%,
   50: 50%,
   75: 75%,
-  100: 100%
+  100: 100%,
+  auto: auto
 ), $sizes);
 // stylelint-enable