gh-80937: Fix memory leak in tkinter createcommand (GH-152294)

gh-80937: Fix memory leak in tkinter createcommand (GH-152294)

A command created with createcommand() held a strong reference to the
interpreter, forming an uncollectable cycle (interpreter -> command ->
interpreter) that kept the interpreter and the callback alive until the
command was removed with deletecommand() or destroy().  The command now
borrows the reference; it cannot outlive the interpreter, which deletes its
commands when finalized.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>