LibJS: Transcode UTF-8 strings to UTF-16 and add UTF-16 accessors

LibJS parses JavaScript as UTF-8, so when creating a string, we must
transcode it to UTF-16 to handle encoded surrogate pairs.

For example, consider the following string:
    "\ud83d\ude00"

The UTF-8 encoding of this surrogate pair is:
    0xf0 0x9f 0x98 0x80

However, LibJS will currently store the two surrogates individually as
UTF-8 encoded bytes, rather than combining the pair:
    0xed 0xa0 0xb8, 0xed 0xb8 0x80

These are not equivalent. So, as String.prototype becomes UTF-16 aware,
this encoding will no longer work for abstractions like strict equality.

Userland/Libraries/LibJS/Runtime/Value.cpp

@@ -9,6 +9,7 @@
 #include <AK/FlyString.h>
 #include <AK/String.h>
 #include <AK/StringBuilder.h>
+#include <AK/Utf16View.h>
 #include <AK/Utf8View.h>
 #include <LibCrypto/BigInt/SignedBigInteger.h>
 #include <LibCrypto/NumberTheory/ModularFunctions.h>
@@ -365,6 +366,18 @@ String Value::to_string(GlobalObject& global_object, bool legacy_null_to_empty_s
     }
 }
 
+Vector<u16> Value::to_utf16_string(GlobalObject& global_object) const
+{
+    if (m_type == Type::String)
+        return m_value.as_string->utf16_string();
+
+    auto utf8_string = to_string(global_object);
+    if (global_object.vm().exception())
+        return {};
+
+    return AK::utf8_to_utf16(utf8_string);
+}
+
 // 7.1.2 ToBoolean ( argument ), https://tc39.es/ecma262/#sec-toboolean
 bool Value::to_boolean() const
 {