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/CMakeLists.txt

@@ -147,6 +147,7 @@ set(SOURCES
     HTML/ImageData.cpp
     HTML/Parser/Entities.cpp
     HTML/Parser/HTMLDocumentParser.cpp
+    HTML/Parser/HTMLEncodingDetection.cpp
     HTML/Parser/HTMLToken.cpp
     HTML/Parser/HTMLTokenizer.cpp
     HTML/Parser/ListOfActiveFormattingElements.cpp