LibJS+LibWeb: Move headers around to allow including Value from Cell

The goal here is to allow Cell::initialize to return a ThrowCompletion,
to handle OOM for example. Cell.h will then need to include Completion.h
which must include Value.h. This currently can't happen because Value.h
includes BigInt.h, which in turn includes Cell.h. So we would have an
include cycle.

This removes BigInt.h from Value.h, as it is forward-declarable (it is
only referred to with a reference or pointer). Then the Value overload
for Cell::Visitor::visit is moved to Cell.h, and missing BigInt.h
includes as peppered as needed.

Userland/Libraries/LibJS/Runtime/NumberConstructor.cpp

@@ -6,6 +6,7 @@
 
 #include <AK/Math.h>
 #include <LibJS/Runtime/AbstractOperations.h>
+#include <LibJS/Runtime/BigInt.h>
 #include <LibJS/Runtime/Error.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/NumberConstructor.h>