Make code.py line number available after safemode

[anecdata]

It would be very helpful to know which line number was executing when a safemode occurred, perhaps with a supervisor attribute.

It's a reasonably-involved process (not beginner-friendly) to make a debug build, collect a backtrace, and decode it. And sometimes the backtrace is corrupted, or otherwise incorrect due to different build parameters with DEBUG=1, or perhaps other reasons.

Related to #1054

Brief Discord discussion here

[tannewt]

Line number of the main file or would we try and keep the filename too? And if we store a string, why not the full traceback? Similar to #7490