[3.12] gh-112672: Fix builtin Tkinter with Tcl 9.0 (GH-112681) (GH-120209)

* Add declaration of Tcl_AppInit(), missing in Tcl 9.0.
* Use Tcl_Size instead of int where needed.

(cherry picked from commit e0799352823289fafb8131341abd751923ee9c08)

Co-authored-by: Christopher Chavez <chrischavez@gmx.us>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>