Add infrastructure to more easily apply visual cue for zoom users enhancement

[owenatgov]

What

Add to the autocomplete's render output to make it easier to apply an enhancement to the GOV.UK Design System's site search, implemented in alphagov/govuk-design-system#4220. These changes would be:

• a wrapper around the ul menu element which we could attach the visual separator to as a pseudo element
• an extra class on the input when a dropdown is visible to make it easier to target changes linked to the visual enhancement

Why

This is in response to alphagov/govuk-design-system#4015, an issue raised by a DAC audit on the GOV.UK Design System website. Our solution would theoretically benefit everyone so it makes sense to move this upstream.

We started this on the autocomplete originally in #753 but opted to rebuild it directly on our website since we'd need to restyle it anyway after launch and this meant we could test it and launch it sooner and in situ. Now that we've done this, we should distribute it more widely.

In addition, adjusting the rendered markup will make this change a lot easier to manage. The styling for this solution using only the input and menu ul is more complex than it needs to be.