LibWeb: Add {de}serialization steps for RegExpObjects

We skip serializing any of the internal state of the Regex<ECMA262>
object, because that state is all computable from the input pattern
and flags. If it turns out that this is really, really slow, we can add
some optimizations to serialize more of the regex parse result.

Tests/LibWeb/Text/input/HTML/StructuredClone-object-primitives.html

@@ -8,6 +8,7 @@
         println(structuredClone(new String("This is a String object")));
         println(structuredClone(BigInt("0x1fffffffffffff")));
         println(structuredClone(Date.UTC(2023, 7, 23)));
+        println(structuredClone(/abc/gimsuy));
 
         try {
             structuredClone(Symbol("foo"));