Optionally emit created for existing files on start up

[davetapley]

It looks like there's some existing appetite for such a feature:

• https://stackoverflow.com/questions/59265504/python-watchdog-process-existing-files-on-startup
• https://stackoverflow.com/questions/66532578/watchdog-create-events-for-existing-files-on-start-up
• https://www.reddit.com/r/learnpython/comments/fz57og/comment/fn34dgk/

My use case:

I have APScheduler jobs defined in files, and the callback to get the job started is the same whether the file is present at start up or created later. So it would simplify things if Watchdog could emit an on_created for files present on startup, and my code didn't need to care whether the file was present at start up or created afterwards.