[refs nhsuk#662] Align back breadcrumb icon with text

This ensures that the back breadcrumb chevron icon is
vertically aligned with the text, even at large font sizes.

packages/components/breadcrumb/_breadcrumb.scss

@@ -100,11 +100,14 @@
     background: url("data:image/svg+xml,%3Csvg class='nhsuk-icon nhsuk-icon__chevron-left' xmlns='http://www.w3.org/2000/svg' fill='%23005eb8' height='24' width='24' viewBox='8 0 24 24' aria-hidden='true'%3E%3Cpath d='M8.5 12c0-.3.1-.5.3-.7l5-5c.4-.4 1-.4 1.4 0s.4 1 0 1.4L10.9 12l4.3 4.3c.4.4.4 1 0 1.4s-1 .4-1.4 0l-5-5c-.2-.2-.3-.4-.3-.7z'%3E%3C/path%3E%3C/svg%3E")
       no-repeat;
     content: "";
+    background-position: left top;
     display: inline-block;
     height: 18px;
     left: 0;
     position: absolute;
-    top: 0;
+    margin: auto;
+    top: -2px;
+    bottom: 2px;
     width: 10px;
   }
 }