With SpiderMonkey 68, it's no longer necessary to mark SpiderMonkey
header files as system headers, and it's also no longer necessary to
include <js-config.h> before any other header files, as SpiderMonkey
has sorted that out internally.

We remove jsapi-wrapper.h and define DEBUG directly in config.h instead
of indirectly in jsapi-wrapper.h via HAVE_SPIDERMONKEY_DEBUG.

This should reduce compile time, on average, because in most .h files
we now only need to include <js/TypeDecls.h>.