cmd/hello: split css and js into separate files (#19771)

Move the inline CSS and JS into separate files to be more friendly
to Content Security Policies. ServeHTTP is updated to serve these
assets from the '/static/' path.

Updates tailscale/corp#32398

Signed-off-by: Noel O'Brien <noel@tailscale.com>

cmd/hello/helloserver/static/script.js

@@ -0,0 +1,12 @@
+(function () {
+    var lastSeen = localStorage.getItem("lastSeen");
+    if (!lastSeen) {
+        document.body.classList.add("animate");
+        window.addEventListener("load", function () {
+            setTimeout(function () {
+                document.body.classList.add("animating");
+                localStorage.setItem("lastSeen", Date.now());
+            }, 100);
+        });
+    }
+})();