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/JsonObject.cpp

@@ -0,0 +1,21 @@
+#include <AK/JsonObject.h>
+#include <AK/StringBuilder.h>
+
+String JsonObject::to_string() const
+{
+    StringBuilder builder;
+    int index = 0;
+    builder.append('{');
+    for_each_member([&] (auto& key, auto& value) {
+        builder.append('"');
+        builder.append(key);
+        builder.append('"');
+        builder.append(':');
+        builder.append(value.to_string());
+        if (index != size() - 1)
+            builder.append(',');
+        ++index;
+    });
+    builder.append('}');
+    return builder.to_string();
+}