Add header-checking function (#9880)

* Add basic header-checking function

* Add Slack notification logic

* Modify Netlify Functions setup

* Rework function logic

* Add missing dependencies

* Remove package-lock.json from Git

* Separate header checking function

* Isolate env check into a separate function

Makefile

@@ -2,6 +2,8 @@ DOCKER       = docker
 HUGO_VERSION = 0.49
 DOCKER_IMAGE = kubernetes-hugo
 DOCKER_RUN   = $(DOCKER) run --rm --interactive --tty --volume $(PWD):/src
+NODE_BIN     = node_modules/.bin
+NETLIFY_FUNC = $(NODE_BIN)/netlify-lambda
 
 .PHONY: all build sass build-preview help serve
 
@@ -16,6 +18,9 @@ build: ## Build site with production settings and put deliverables in ./public
 build-preview: ## Build site with drafts and future posts enabled
 	hugo -D -F
 
+functions-build:
+	$(NETLIFY_FUNC) build functions-src
+
 check-headers-file:
 	scripts/check-headers-file.sh