add command to build examples to Makefile

Makefile

@@ -2,3 +2,10 @@
 test:
 	@PATH=$(CURDIR)/misc/wasm:$(PATH) GOOS=js GOARCH=wasm go test ./...
 
+.PHONY: build-examples
+build-examples:
+	for dir in $(shell find ./_examples -maxdepth 1 -type d); do \
+		if [ $$dir = "./_examples" ]; then continue; fi; \
+		cd $$dir && GOOS=js GOARCH=wasm go build -o ./build/app.wasm; \
+		cd ../../; \
+	done