gh-71592: Add ability to trace Tcl commands executed by Tkinter (GH-118291)

This is an experimental feature, for internal use.

Setting tkinter._debug = True before creating the root window enables
printing every executed Tcl command (or a Tcl command equivalent to the
used Tcl C API).

This will help to convert a Tkinter example into Tcl script to check
whether the issue is caused by Tkinter or exists in the underlying Tcl/Tk
library.