Track Lunr search queries and search-result clicks

[chalin]

• Closes #1518
• Contributes to #1289
• Adds an event handler to send search terms as a search event to GA4 with the search term as an attribute, at no more than the debounce rate.
• Adds a click event handler to each search-result item. When a user clicks on a search result, a click event is sent to GA4 with the destination link and associated search term as attributes.

A note about the deploy preview: the site preview will allow you to confirm that search is still functional, but it won't generate GA4 events, since analytics is only enabled in production. When _debug is enabled (which it was for my original commit, but has since been disabled), you can at least see that the tracking functions get called, like this:

trackSearchEvent: searchTerm = installation
trackSearchResultClick: searchTerm = installation, link = https://deploy-preview-1519--vitess.netlify.app/docs/17.0/get-started/local-mac/

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I've locally built a "production" version of the site, and can confirm that GA4 is receiving both search and click (related to search) events. Notice the search_term attribute in both events:

Event	Event attributes
Notice the search event with event count of 1:
Oops forgot to get a click-event screenshot

/cc @harshit-gangal @thisisobate @nate-double-u

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