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.h

@@ -246,6 +246,7 @@ public:
     u64 encoded() const { return m_value.encoded; }
 
     String to_string(GlobalObject&, bool legacy_null_to_empty_string = false) const;
+    Vector<u16> to_utf16_string(GlobalObject&) const;
     PrimitiveString* to_primitive_string(GlobalObject&);
     Value to_primitive(GlobalObject&, PreferredType preferred_type = PreferredType::Default) const;
     Object* to_object(GlobalObject&) const;