Add easter egg achievement

src/utilities/easteregg_handler.ts

@@ -0,0 +1,28 @@
+const EASTER_EGGS = ["cookie", "future"];
+
+export function addFoundEasterEgg(name:string):void {
+    if (!EASTER_EGGS.includes(name)) { return; }
+
+    let found_eastereggs = new Set( JSON.parse(localStorage.getItem("eastereggs") ?? "[]") );
+    const is_new = !found_eastereggs.has(name);
+
+    found_eastereggs.add(name);
+    localStorage.setItem("eastereggs", JSON.stringify([...found_eastereggs]));
+
+    if (is_new) {
+        // To trigger the banner
+        document.dispatchEvent(new CustomEvent("easteregg", { detail: name }));
+    }
+}
+
+export function getFoundEasterEggs():string[] {
+    return JSON.parse(localStorage.getItem("eastereggs") ?? "[]");
+}
+
+export function getFoundEasterEggsCount():number {
+    return getFoundEasterEggs().length;
+}
+
+export function getTotalEasterEggsCount():number {
+    return EASTER_EGGS.length;
+}