Migrate demo to gulp (#3152)

* Migrate demo to gulp

* Tweak demo migration to gulp

* Feature detect demo

* Gen icons

build-scripts/gulp/demo.js

@@ -0,0 +1,36 @@
+// Run HA develop mode
+const gulp = require("gulp");
+
+require("./clean.js");
+require("./translations.js");
+require("./gen-icons.js");
+require("./gather-static.js");
+require("./webpack.js");
+require("./service-worker.js");
+require("./entry-html.js");
+
+gulp.task(
+  "develop-demo",
+  gulp.series(
+    async function setEnv() {
+      process.env.NODE_ENV = "development";
+    },
+    "clean-demo",
+    gulp.parallel("gen-icons", "gen-icons-demo", "build-translations"),
+    "copy-static-demo",
+    "webpack-dev-server-demo"
+  )
+);
+
+gulp.task(
+  "build-demo",
+  gulp.series(
+    async function setEnv() {
+      process.env.NODE_ENV = "production";
+    },
+    "clean-demo",
+    gulp.parallel("gen-icons", "gen-icons-demo", "build-translations"),
+    "copy-static-demo",
+    "webpack-prod-demo"
+  )
+);