LibMarkdown: Recognize and handle comments

This also improves Commonmark coverage, e.g. it fixes tests
HTML_blocks_ex179_2894..2906 and Lists_ex308_5439..5457.

In other words, we go from 271 out of 652 to 273 out of 652.

Userland/Libraries/LibMarkdown/CommentBlock.cpp

@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2021, Ben Wiederhake <BenWiederhake.GitHub@gmx.de>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/StringBuilder.h>
+#include <LibMarkdown/CommentBlock.h>
+#include <LibMarkdown/Visitor.h>
+
+namespace Markdown {
+
+String CommentBlock::render_to_html(bool) const
+{
+    StringBuilder builder;
+
+    builder.append("<!--");
+    builder.append(escape_html_entities(m_comment));
+    // TODO: This is probably incorrect, because we technically need to escape "--" in some form. However, Browser does not care about this.
+    builder.append("-->\n");
+
+    return builder.build();
+}
+
+String CommentBlock::render_for_terminal(size_t) const
+{
+    return "";
+}
+
+RecursionDecision CommentBlock::walk(Visitor& visitor) const
+{
+    RecursionDecision rd = visitor.visit(*this);
+    if (rd != RecursionDecision::Recurse)
+        return rd;
+
+    // Normalize return value.
+    return RecursionDecision::Continue;
+}
+
+OwnPtr<CommentBlock> CommentBlock::parse(LineIterator& lines)
+{
+    if (lines.is_end())
+        return {};
+
+    constexpr auto comment_start = "<!--"sv;
+    constexpr auto comment_end = "-->"sv;
+
+    StringView line = *lines;
+    if (!line.starts_with(comment_start))
+        return {};
+    line = line.substring_view(comment_start.length());
+
+    StringBuilder builder;
+
+    while (true) {
+        // Invariant: At the beginning of the loop, `line` is valid and should be added the the builder.
+        bool ends_here = line.ends_with(comment_end);
+        if (ends_here)
+            line = line.substring_view(0, line.length() - comment_end.length());
+        builder.append(line);
+        if (!ends_here)
+            builder.append('\n');
+
+        ++lines;
+        if (lines.is_end() || ends_here) {
+            break;
+        }
+        line = *lines;
+    }
+
+    return make<CommentBlock>(builder.build());
+}
+
+}