There's a quirk in HTML where the parser should ignore any line feed
character immediately following a `pre` or `textarea` start tag.
This was working fine when we could peek ahead in the input stream and
see the next token, but didn't work in character-at-a-time parsing with
document.write().
This commit adds the "can ignore next line feed character" as a parser
flag that is maintained across invocations, making it work in this
parsing mode as well.
20 new passes in WPT/html/syntax/parsing/ :^)
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
