This commit allows building js.cpp on windows. The repl functionality is
ifdef'ed out. To decrease the number of ifdefs the code that runs the
repl on Linux was moved into one place. Some globals that are unused as
a result of that are markes maybe_unused. The following commit enables
building and testing js in cmake for windows.
The special empty value (that we use for array holes, Optional<Value>
when empty and a few other other placeholder/sentinel tasks) still
exists, but you now create one via JS::js_special_empty_value() and
check for it with Value::is_special_empty_value().
The main idea here is to make it very unlikely to accidentally create an
unexpected special empty value.
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
