job.sh: Add synchronization helpers

[nhooyr]

Want to be able to make runjob be able to limit the number of concurrent processes to x.

And for job's to be able to wait on an already running job before starting the very same job.

e.g. if you have two jobs to run fmt and lint and both depend on yarn being ran first to fetch dependencies.

Will require a dependency on inotifywait for linux and fswatch for macOS.

[nhooyr]

When I add this I should remember to add something that also installs CI deps like prettier/goimports/pandoc automatically but serialized.

[nhooyr]

And then remember to remove from ci setup scripts.

[nhooyr]

lockfile_wait

each job represents a lockfile in $TMPDIR.

Ties into #19

[nhooyr]

No need for fsnotify, just poll.

[nhooyr]

also automatically gets package.json and go module dependencies as needed instead of having to do it in a separate step before.