LibJS: Implement the Intl.Segmenter FindBoundary AO

Author: https://github.com/IdanHo
Commit: cea6c81c77
Pull-request: https://github.com/SerenityOS/serenity/pull/12229
Reviewed-by: https://github.com/trflynn89 ✅

Userland/Libraries/LibJS/Runtime/Intl/Segmenter.h

@@ -36,4 +36,10 @@ private:
     SegmenterGranularity m_segmenter_granularity { SegmenterGranularity::Grapheme }; // [[SegmenterGranularity]]
 };
 
+enum class Direction {
+    Before,
+    After,
+};
+double find_boundary(Segmenter const&, Utf16View const&, double start_index, Direction, Optional<Vector<size_t>>& boundaries_cache);
+
 }