Center text and icons vertically in the option select component

The font size of this element increased with the typography changes
deployed in a recent update to `govuk-frontend`. This means that the
up/down icons no longer align vertically. This PR makes a minor tweak
to the position of the icons for better vertical balance.

This also supports the same change made in `finder-frontend` where
search filter titles had the same tweak to icon positions. See the PR
at alphagov/finder-frontend#3334.

Note that a refactor of this element could use a better solution
such as flexbox, rather than using absolutely positioned icons
and magic numbers.

app/assets/stylesheets/govuk_publishing_components/components/_option-select.scss

@@ -77,7 +77,7 @@
 .gem-c-option-select__icon {
   display: none;
   position: absolute;
-  top: 0;
+  top: 3px;
   left: 9px;
   width: 30px;
   height: 40px;