add category to customSearch event (#8613)

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### WHY are these changes introduced?

Missing `category` for analytics reports to have better filtering

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### WHAT is this pull request doing?

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changes in the style guide

Author: Yuraima Estevez

.changeset/sharp-bulldogs-repair.md

@@ -0,0 +1,5 @@
+---
+'polaris.shopify.com': minor
+---
+
+Added 'engagement' category to search event

polaris.shopify.com/src/components/GlobalSearch/GlobalSearch.tsx

@@ -52,6 +52,7 @@ function captureSearchEvent(
     searchTerm,
     resultRank,
     selectedResult,
+    category: 'engagement',
   };
 
   if (searchTerm) {