IDLE: rename stackviewer module to stackbrowser

[terryjreedy]

Bug report

The stackviewer module contains the StackBrowser class and the recently renamed _stackbrowser htest function. StackBrowser is used to view the stack after an exception. The debugger module contains the StackViewer class and a stackviewer instance thereof. These are used to view the stack while debugging with the IDLE debugger. The easiest fix for this name confusion is to rename the StackBrowser-containing file/module to stackbrowser.

Linked PRs

• gh-112695: Rename idlelib/stackviewer module to stackbrowser #121420