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>

Makefile

@@ -155,6 +155,7 @@ help:
 	@echo " - build                            build everything"
 	@echo " - frontend                         build frontend files"
 	@echo " - backend                          build backend files"
+	@echo " - watch                            watch everything and continuously rebuild"
 	@echo " - watch-frontend                   watch frontend files and continuously rebuild"
 	@echo " - watch-backend                    watch backend files and continuously rebuild"
 	@echo " - clean                            delete backend and integration files"
@@ -316,6 +317,10 @@ lint-frontend: node_modules
 .PHONY: lint-backend
 lint-backend: golangci-lint revive vet
 
+.PHONY: watch
+watch:
+	bash tools/watch.sh
+
 .PHONY: watch-frontend
 watch-frontend: node-check $(FOMANTIC_DEST) node_modules
 	rm -rf $(WEBPACK_DEST_ENTRIES)