LibWeb: Implement encoding sniffing algorithm

This patch implements the HTML specification's "encoding sniffing
algorithm", which is used when no encoding can be obtained from the
Content-Type header (either because it doesn't contain a charset=...)
value or the file has not been opened via HTTP (as with local files).

It also modifies the creator of the HTMLDocumentParser to use the new
HTMLDocumentParser::create_with_uncertain_encoding static method, which
runs the encoding sniffing algorithm before instantiating the parser.

This now allows us to load local HTML pages (or remote pages without a
charset specified in the 'Content-Type' header) with a non-UTF-8
encoding such as 'windows-1252'. This would previously crash the
browser. :^)

Userland/Libraries/LibWeb/HTML/Parser/HTMLEncodingDetection.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2021, Max Wipfli <mail@maxwipfli.ch>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <AK/String.h>
+#include <LibWeb/DOM/Attribute.h>
+
+namespace Web::HTML {
+
+bool prescan_should_abort(const ByteBuffer& input, const size_t& position);
+bool prescan_is_whitespace_or_slash(const u8& byte);
+bool prescan_skip_whitespace_and_slashes(const ByteBuffer& input, size_t& position);
+Optional<Attribute> prescan_get_attribute(const ByteBuffer& input, size_t& position);
+Optional<String> run_prescan_byte_stream_algorithm(const ByteBuffer& input);
+String run_encoding_sniffing_algorithm(const ByteBuffer& input);
+
+}