AK+LibJS: Do not set UTF-16 code point length to its code unit length

Author: https://github.com/trflynn89
Commit: efa9737cf7
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5214
Reviewed-by: https://github.com/gmta ✅

AK/Utf16View.h

@@ -113,6 +113,13 @@ public:
     size_t length_in_code_units() const { return m_code_units.size(); }
     size_t length_in_code_points() const;
 
+    Optional<size_t> length_in_code_points_if_known() const
+    {
+        if (m_length_in_code_points == NumericLimits<size_t>::max())
+            return {};
+        return m_length_in_code_points;
+    }
+
     Utf16CodePointIterator begin() const { return { begin_ptr(), m_code_units.size() }; }
     Utf16CodePointIterator end() const { return { end_ptr(), 0 }; }