Use `$EPOCHREALTIME` (or `$SECONDS`) built-in variable provided by Bash instead of `date +%s`. We're only measuing the difference in seconds, so avoid both the binary invocation as well as the subshell.
Alsö, Reduce environmental pollution by not exporting every variable, and unsetting when done.
Change variable names to match lib/command-duration
Remove `preexec_return_notification()` in favor of `lib/command-duration`'s `_command_duration_pre_exec()`.
This should now use the same preexec hook and variables as the theme library `command_duration`.
tests: handle nanoseconds
6ca10cf84c