This porting effort makes it pretty clear we will want a UTF-16-aware
GenericLexer. But for now, we can actually make ASCII assumptions about
what we are parsing, and act accordingly.
Which has an optmization if both size of the string being passed
through are FlyStrings, which actually ends up being the case
in some places during selector matching comparing attribute names.
Instead of maintaining more overloads of
Infra::is_ascii_case_insensitive_match, switch
everything over to equals_ignoring_ascii_case instead.
isomorphic encoding a value that has already been encoded will
result in garbage data. `response_headers` is already encoded in
ISO-8859-1/latin1, we cannot use `from_string_pair`, as it triggers
ISO-8859-1/latin1 encoding.
Follow-up of https://github.com/LadybirdBrowser/ladybird/pull/1893
