LibWeb: Begin work on a spec-compliant HTML parser

In order to actually view the web as it is, we're gonna need a proper
HTML parser. So let's build one!

This patch introduces the Web::HTMLTokenizer class, which currently
operates on a StringView input stream where it fetches (ASCII only atm)
codepoints and tokenizes acccording to the HTML spec tokenization algo.

The tokenizer state machine looks a bit weird but is written in a way
that tries to mimic the spec as closely as possible, in order to make
development easier and bugs less likely.

This initial version is far from finished, but it can parse a trivial
document with a DOCTYPE and open/close tags. :^)

Libraries/LibWeb/Parser/HTMLToken.h

@@ -0,0 +1,76 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/String.h>
+#include <AK/StringBuilder.h>
+#include <AK/Types.h>
+#include <AK/Vector.h>
+#include <LibWeb/DOM/Attribute.h>
+
+namespace Web {
+
+class HTMLToken {
+    friend class HTMLTokenizer;
+
+public:
+    enum class Type {
+        DOCTYPE,
+        StartTag,
+        EndTag,
+        Comment,
+        Character,
+        EndOfFile,
+    };
+
+    Type type() const { return m_type; }
+
+private:
+    Type m_type;
+
+    // Type::DOCTYPE
+    struct {
+        StringBuilder name;
+        StringBuilder public_identifier;
+        StringBuilder system_public_identifier;
+        bool force_quirks { false };
+    } m_doctype;
+
+    // Type::StartTag
+    // Type::EndTag
+    struct {
+        StringBuilder tag_name;
+        bool self_closing { false };
+        Vector<Attribute> attributes;
+    } m_tag;
+
+    struct {
+        StringBuilder data;
+    } m_comment_or_character;
+};
+
+}