Add a tagged template literal function stanza

For creating XML stanzas from template literals.

src/headless/core.js

@@ -762,6 +762,7 @@ Object.assign(converse, {
         log,
         sizzle,
         sprintf,
+        stanza: u.stanza,
         u,
     }
 });

src/headless/utils/stanza.js

@@ -0,0 +1,21 @@
+const parser = new DOMParser();
+const parserErrorNS = parser.parseFromString('invalid', 'text/xml')
+                            .getElementsByTagName("parsererror")[0].namespaceURI;
+
+export function toStanza (string) {
+    const node = parser.parseFromString(string, "text/xml");
+    if (node.getElementsByTagNameNS(parserErrorNS, 'parsererror').length) {
+        throw new Error(`Parser Error: ${string}`);
+    }
+    return node.firstElementChild;
+}
+
+
+export function stanza (strings, ...values) {
+    return toStanza(
+        strings.reduce((acc, str) => {
+            const idx = strings.indexOf(str);
+            return acc + str + (values.length > idx ? values[idx] : '')
+        }, '')
+    );
+}