[IMP] app: allow to instantiate templates lazily

With this commit, the templates that are given while instantiating the
App class can be an object with templates not yet parsed (i.e. string,
not Document). This allow to instantiate the App class with templates
that are not yet parsed, and these templates will be parsed only when
needed.

Part of task-id 3601257

src/runtime/template_set.ts

@@ -41,7 +41,7 @@ export interface TemplateSetConfig {
   dev?: boolean;
   translatableAttributes?: string[];
   translateFn?: (s: string) => string;
-  templates?: string | Document;
+  templates?: string | Document | Record<string, string>;
 }
 
 export class TemplateSet {
@@ -60,7 +60,13 @@ export class TemplateSet {
     this.translateFn = config.translateFn;
     this.translatableAttributes = config.translatableAttributes;
     if (config.templates) {
-      this.addTemplates(config.templates);
+      if (config.templates instanceof Document || typeof config.templates === "string") {
+        this.addTemplates(config.templates);
+      } else {
+        for (const name in config.templates) {
+          this.addTemplate(name, config.templates[name]);
+        }
+      }
     }
   }