Add run once mode to `edgedb watch`

[ssendev]

Add a command line option to exit edgedb watch after running once. Which means it's actually not watching so maybe add it as another subcommand

This would be useful in scripts and is related to #1099

Currently can be faked with edgedb watch 2>&1 | while read line; do; echo $line; echo "x$line" | grep -q 'Initialized. Monitoring' && pkill --parent $$ -x edgedb; done but is pretty unclean

[raddevon]

This is available as the --dev-mode option on edgedb migrate/edgedb migration apply.