[Feature Request] Autoupdate like zgenom

[QuarticCat]

Reference: https://github.com/jandamm/zgenom#run-updates-automatically

I saw zgenom record update time in the file, but we actually don't have to do this. We can simply use mtime. An oversimplified example is given below:

_qc_last_update=(~zdot/.zcomet/update(Nm-7))
if [[ -z $_qc_last_update ]] {
    touch ~zdot/.zcomet/update
    zcomet self-update
    zcomet update
}