Added header

header.js

@@ -0,0 +1,31 @@
+class SiteHeader extends HTMLElement {
+    connectedCallback() {
+        fetch("site-header.html")
+            .then(response => response.text())
+            .then(html => {
+                this.innerHTML = html;
+                detectPage(); 
+            });
+    }
+}
+customElements.define('site-header', SiteHeader);
+
+function detectPage() {
+    var pageId = document.title.toLowerCase().replace(/ /g, "-"); 
+    var element = document.getElementById(pageId);
+    if (element) {
+        element.className = "active";
+    } else {
+        console.warn("Could not find a navigation link matching the ID: " + pageId);
+    }
+}
+
+function toggleMobileMenu() {
+    var nav = document.getElementById("myTopnav");
+    
+    if (nav.className === "topnav") {
+        nav.className += " responsive";
+    } else {
+        nav.className = "topnav";
+    }
+}