Add 'make watch' (#12636)

* Add 'make watch'

This combines frontend and backend watch into a single command that runs
them in parallel on on SIGINT terminates both.

Termination is not super-clean but I guess it does not have to.

* move to tools/, trap more signals, remove gnu-specific flag

* simplify

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

tools/watch.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+set -euo pipefail
+
+make watch-frontend &
+make watch-backend &
+
+trap 'kill $(jobs -p)' EXIT
+wait