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/JsonValue.h

@@ -0,0 +1,53 @@
+#pragma once
+
+#include <AK/AKString.h>
+
+class JsonArray;
+class JsonObject;
+
+class JsonValue {
+public:
+    enum class Type {
+        Undefined,
+        Null,
+        Int,
+        Double,
+        Bool,
+        String,
+        Array,
+        Object,
+    };
+
+    explicit JsonValue(Type = Type::Null);
+    ~JsonValue() { clear(); }
+
+    JsonValue(const JsonValue&);
+    JsonValue(JsonValue&&);
+
+    JsonValue& operator=(const JsonValue&);
+    JsonValue& operator=(JsonValue&&);
+
+    JsonValue(int);
+    JsonValue(double);
+    JsonValue(bool);
+    JsonValue(const String&);
+    JsonValue(const JsonArray&);
+    JsonValue(const JsonObject&);
+
+    String to_string() const;
+
+private:
+    void clear();
+    void copy_from(const JsonValue&);
+
+    Type m_type { Type::Undefined };
+
+    union {
+        StringImpl* as_string { nullptr };
+        JsonArray* as_array;
+        JsonObject* as_object;
+        double as_double;
+        int as_int;
+        bool as_bool;
+    } m_value;
+};