gh-157762: Fix Alt-2 zoom height binding in IDLE tree windows

[lazerg]

ScrolledCanvas binds Alt-2 to zoom_height, which called idlelib.zoomheight.zoom_height. That module-level function went away with the Zoom Height rework in 2019, so Alt-2 in Path Browser, Module Browser or the stack viewer raises AttributeError out of the Tk callback.

ZoomHeight only needs the toplevel of the window it zooms, so ScrolledCanvas keeps one and calls ZoomHeight(self).zoom_height().

Fixes #157762

• Issue: IDLE: ScrolledCanvas.zoom_height calls idlelib.zoomheight.zoom_height, which has not existed since 2019 #157762

[terryjreedy]

Lazizbek: thank you for the PR. For reasons given on the issue, I am putting the issue and PR on hold.