Defer Instant Search styles unless we're opening the modal immediately

We received some user feedback that Google Pagespeed Insights flags the Instant Search stylesheet as a performance concern (pcNPJE-cq-p2).

One option might be to defer the loading of Instant Search styles unless the query string contains a search query (`?s=`):

https://web.dev/defer-non-critical-css/


No file or test is named. Start by locating the Instant Search stylesheet and the code that checks the search query string; confirm the stylesheet is deferred when no ?s= query is present and available when the modal opens immediately.

Written by the indexing model from the issue text.