JSON: Add JSON serializers

These are two new types that allow serializing JSON on-the-fly
as it's generated, without building the whole JSON in memory
first.

AK/JsonArraySerializer.h

@@ -0,0 +1,65 @@
+#pragma once
+
+#include <AK/JsonValue.h>
+
+namespace AK {
+
+template<typename Builder>
+class JsonObjectSerializer;
+
+template<typename Builder>
+class JsonArraySerializer {
+public:
+    explicit JsonArraySerializer(Builder& builder)
+        : m_builder(builder)
+    {
+        m_builder.append('[');
+    }
+
+    JsonArraySerializer(const JsonArraySerializer&) = delete;
+    JsonArraySerializer(JsonArraySerializer&&) = delete;
+
+    ~JsonArraySerializer()
+    {
+        if (!m_finished)
+            finish();
+    }
+
+    void add(const JsonValue& value)
+    {
+        begin_item();
+        value.serialize(m_builder);
+    }
+
+    JsonArraySerializer<Builder> add_array()
+    {
+        begin_item();
+        return JsonArraySerializer(m_builder);
+    }
+
+    // Implemented in JsonObjectSerializer.h
+    JsonObjectSerializer<Builder> add_object();
+
+    void finish()
+    {
+        ASSERT(!m_finished);
+        m_finished = true;
+        m_builder.append(']');
+    }
+
+private:
+    void begin_item()
+    {
+        if (!m_empty)
+            m_builder.append(',');
+        m_empty = false;
+    }
+
+    Builder& m_builder;
+    bool m_empty { true };
+    bool m_finished { false };
+};
+
+}
+
+using AK::JsonArraySerializer;