AK+Everywhere: Recognise that surrogates in utf16 aren't all that common

For the slight cost of counting code points when converting between
encodings and a teeny bit of memory, this commit adds a fast path for
all-happy utf-16 substrings and code point operations.

This seems to be a significant chunk of time spent in many regex
benchmarks.

Libraries/LibJS/Runtime/Utf16String.h

@@ -48,6 +48,7 @@ private:
     mutable bool m_has_hash { false };
     mutable u32 m_hash { 0 };
     Utf16Data m_string;
+    Utf16View m_cached_view { m_string.span() };
 };
 
 }