gh-79871: IDLE - Fix and test debugger module (#11451)

Add docstrings to the debugger module. Fix two bugs: initialize Idb.botframe (should be in Bdb); In Idb.in_rpc_code, check whether prev_frame is None before trying to use it. Make other code changes.

Expand test_debugger coverage from 19% to 66%.
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Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>