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.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <AK/JsonValue.h>
+#include <AK/Vector.h>
+
+class JsonArray {
+public:
+    JsonArray() {}
+    ~JsonArray() {}
+
+    int size() const { return m_values.size(); }
+    bool is_empty() const { return m_values.is_empty(); }
+
+    const JsonValue& at(int index) const { return m_values.at(index); }
+    const JsonValue& operator[](int index) const { return at(index); }
+
+    void clear() { m_values.clear(); }
+    void append(const JsonValue& value) { m_values.append(value); }
+
+    String to_string() const;
+
+private:
+    Vector<JsonValue> m_values;
+};