LibMarkdown: Refactor Document's parser into ContainerBlock

This will better allow us too do things like have Lists and blockquotes
support multiple blocks.

Userland/Libraries/LibMarkdown/ContainerBlock.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2021, Peter Elliott <pelliott@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullOwnPtrVector.h>
+#include <AK/OwnPtr.h>
+#include <AK/String.h>
+#include <LibMarkdown/Block.h>
+
+namespace Markdown {
+
+class ContainerBlock final : public Block {
+public:
+    ContainerBlock(NonnullOwnPtrVector<Block> blocks)
+        : m_blocks(move(blocks))
+    {
+    }
+
+    virtual ~ContainerBlock() override { }
+
+    virtual String render_to_html() const override;
+    virtual String render_for_terminal(size_t view_width = 0) const override;
+
+    static OwnPtr<ContainerBlock> parse(Vector<StringView>::ConstIterator& lines);
+
+private:
+    NonnullOwnPtrVector<Block> m_blocks;
+};
+
+}