Avoid defining unneeded variables on Cygw32

Avoid defining unneeded variables on Cygw32

lisp/term/w32-win.el contained definitions of variables which
were not used on Cygw32.  It is now to split into two files,
common part and Windows native build specific part. The latter
is a new file, lisp/term/w32-nt.el.
* src/image.c (Qlibpng_version, Qlibgif_version)
(Qlibjpeg_version): Don't define on Cygw32 build.
* src/treesit.c (Qtree_sitter__library_abi): Ditto.

* lisp/term/w32-win.el (dynamic-library-alist, libpng-version)
(libgif-version, libjpeg-version, libgnutls-version)
(tree-sitter--library-abi, gui-backend-set-selection)
(gui-backend-get-selection, gui-backend-selection-owner-p)
(gui-selection-exists-p): Moved to lisp/term/w32-nt.el.
* lisp/term/w32-nt.el: New file, separated Windows native build
specific part from lisp/term/w32-win.el.
* lisp/loadup.el: Load term/w32-nt.el on Windows native build.
(Bug#75926)

(cherry picked from commit 64b3ad5c763ed3d2a9978fce06c0ccbdd085fe35)