devenv: have bra watch attempt graceful shutdown (#19857)

This is useful when working with backend plugins and grafana at the same time. Makes it so Grafana has a chance to shutdown the external process. An os.Interrupt signal is first sent, then after a five second wait a a force kill will be sent if the process is still running.

.bra.toml

@@ -17,3 +17,5 @@ cmds = [
   ["go", "run", "-mod=vendor", "build.go", "-dev", "build-server"],
   ["go", "run", "-mod=vendor", "build.go", "-dev", "build-server"],
 	["./bin/grafana-server", "-packaging=dev", "cfg:app_mode=development"]
 	["./bin/grafana-server", "-packaging=dev", "cfg:app_mode=development"]
 ]
 ]
+interrupt_timout = 5
+graceful_kill = true