[polari.shopify.com] Fix issue that caused site search to re-open

[martenbjork]

Changes the event handler for the global search. It fixes a pesky focus bug, but makes the search feel a tiny bit less snappy which we can address later.

OK, I've narrow this one down to this:

• When you press enter, the keydown event closes the modal and returns focus to the previously focused element (a11y best practice)
• The onkeyup event immediately fires on that element causing the modal to re-open

You can prove this by manually setting the focus on the dark mode switcher, then opening the global search using the / key. When you press enter on a result, the search will disappear (onkeydown), focus will return to the dark mode switcher, and the site will switch to light/dark mode (onkeyup).

Fixes #6638

[SeanyB]

🎩 looks good! I see the problem no longer occurs when focus is on the search box.