Fix warning text if border-box sizing applied globally

If a user applies a global box-sizing reset [1], the warning text icon is 'squished' horizontally as the border is 'accounted for' within the 29px min-width.

Explicitly use border-box sizing, as this means that the min-width reflects the actual desired width of the icon, rather than having to factor the border width in separately. As the box-sizing model is now set explicitly, it should display correctly whether the reset is applied or not.

Increase the min-width and min-height of the icon to include the border (3px border * 2 in each direction = 6px).

[1]: https://css-tricks.com/inheriting-box-sizing-probably-slightly-better-best-practice/

src/govuk/components/warning-text/_index.scss

@@ -13,13 +13,15 @@
   .govuk-warning-text__icon {
     @include govuk-font($size: false, $weight: bold);
 
+    box-sizing: border-box;
+
     display: inline-block;
 
     position: absolute;
     left: 0;
 
-    min-width: 29px;
-    min-height: 29px;
+    min-width: 35px;
+    min-height: 35px;
     margin-top: -7px;
 
     @include govuk-media-query($from: tablet) {