Enable `MICROPY_PYEXEC_ENABLE_EXIT_CODE_HANDLING` to propagate `sys.exit()`
exit codes properly. Update `convert_pyexec_result()` to handle return
values where pyexec returns the exit code with `PYEXEC_FORCED_EXIT` flag
set for `SystemExit`. Extract the exit code from the lower 8 bits when the
flag is set, otherwise return as-is (0 for success, 1 for exception).
Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>