LibPDF: Create basic object structure

This commit is the start of LibPDF, and introduces some basic structure
objects. This emulates LibJS's Value structure, where Value is a simple
class that can contain a pointer to a more complex Object class with
more data. All of the basic PDF objects have a representation.

Userland/Libraries/LibPDF/Forward.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2021, Matthew Olsson <mattco@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace PDF {
+
+class Document;
+class Object;
+
+#define ENUMERATE_DIRECT_OBJECT_TYPES(V) \
+    V(StringObject, string)              \
+    V(NameObject, name)                  \
+    V(ArrayObject, array)                \
+    V(DictObject, dict)                  \
+    V(StreamObject, stream)              \
+    V(IndirectValue, indirect_value)
+
+#define ENUMERATE_OBJECT_TYPES(V)    \
+    ENUMERATE_DIRECT_OBJECT_TYPES(V) \
+    V(IndirectValueRef, indirect_value_ref)
+
+#define FORWARD_DECL(class_name, _) class class_name;
+ENUMERATE_OBJECT_TYPES(FORWARD_DECL)
+#undef FORWARD_DECL
+
+template<typename T>
+concept IsObject = IsBaseOf<Object, T>;
+
+}