LibWeb: Implement tokenization newline preprocessing

Newline normalization will replace \r and \r\n with \n.

The spec specifically states
> Before the tokenization stage, the input stream must be preprocessed
> by normalizing newlines.
wheras this is implemented the processing during the tokenization
itself.

This should still exhibit the same behaviour, while keeping the
tokenization logic in the same place.

Userland/Libraries/LibWeb/HTML/Parser/HTMLTokenizer.cpp

@@ -187,9 +187,25 @@ Optional<u32> HTMLTokenizer::next_code_point()
 {
  if (m_utf8_iterator == m_utf8_view.end())
  return {};
- skip(1);
- dbgln_if(TOKENIZER_TRACE_DEBUG, "(Tokenizer) Next code_point: {}", (char)*m_prev_utf8_iterator);
- return *m_prev_utf8_iterator;
+
+ u32 code_point;
+ // https://html.spec.whatwg.org/multipage/parsing.html#preprocessing-the-input-stream:tokenization
+ // https://infra.spec.whatwg.org/#normalize-newlines
+ if (peek_code_point(0).value_or(0) == '\r' && peek_code_point(1).value_or(0) == '\n') {
+ // replace every U+000D CR U+000A LF code point pair with a single U+000A LF code point,
+ skip(2);
+ code_point = '\n';
+ } else if (peek_code_point(0).value_or(0) == '\r') {
+ // replace every remaining U+000D CR code point with a U+000A LF code point.
+ skip(1);
+ code_point = '\n';
+ } else {
+ skip(1);
+ code_point = *m_prev_utf8_iterator;
+ }
+
+ dbgln_if(TOKENIZER_TRACE_DEBUG, "(Tokenizer) Next code_point: {}", code_point);
+ return code_point;
 }
 
 void HTMLTokenizer::skip(size_t count)