AK: Add some classes for JSON encoding.

This patch adds JsonValue, JsonObject and JsonArray. You can use them to
build up a JsonObject and then serialize it to a string via to_string().

This patch only implements encoding, no decoding yet.

AK/JsonArray.cpp

@@ -0,0 +1,15 @@
+#include <AK/JsonArray.h>
+#include <AK/StringBuilder.h>
+
+String JsonArray::to_string() const
+{
+    StringBuilder builder;
+    builder.append('[');
+    for (int i = 0; i < m_values.size(); ++i) {
+        builder.append(m_values[i].to_string());
+        if (i != size() - 1)
+            builder.append(',');
+    }
+    builder.append(']');
+    return builder.to_string();
+}