[IMP] component tags: allow injecting values in xml tag string

With this commit, we can now do this:

      const SUBTEMPLATE = xml`<span><t t-esc="state.n"/></span>`
      class Parent extends Widget {
          static template = xml`<div><t t-call="${SUBTEMPLATE}"/></div>`
          state = {n: 42};
      }

src/tags.ts

@@ -19,8 +19,9 @@ import { QWeb } from "./qweb/index";
  *   }
  * ```
  */
-export function xml(strings) {
+export function xml(strings, ...args) {
   const name = `__template__${QWeb.nextId++}`;
-  QWeb.registerTemplate(name, strings[0]);
+  const value = String.raw(strings, ...args);
+  QWeb.registerTemplate(name, value);
   return name;
 }