By default, use more traditional linking pattern with meson.
The janet.exe will now link to janet-x.x.dll on windows (and
similar for linux/posix) when built with meson. This is slightly
less efficient and means that janet.exe built this way is no longer
standalone (you would need to move the dll along with the exe), but
plays better with most build systems.
One is a way to export symbols, the other a way to reference
API functions. Also include prebuilt dlljanet.dll and dlljanet.lib
for windows to save people the trouble of compiling janet.c themselves.
JPM and related functionality has been moved to it's own repository
and will be versioned separately from Janet. The distribution process
could later be modified to bundle a version of jpm with Janet but this
is perhaps not needed.
