Re-factor DefaultExternalServices into a regular class, without static methods

The `DefaultExternalServices` code, which is used to provide build-specific functionality, is very old. This results in a pattern where we first initialize `PDFViewerApplication.externalServices` and then *override* it for the different builds.

By converting `DefaultExternalServices` into a "regular" class, and leveraging import maps, we can directly initialize the correct instance depending on the build.

web/external_services.js

@@ -0,0 +1,46 @@
+/* Copyright 2024 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+class BaseExternalServices {
+  constructor() {
+    if (this.constructor === BaseExternalServices) {
+      throw new Error("Cannot initialize BaseExternalServices.");
+    }
+  }
+
+  updateFindControlState(data) {}
+
+  updateFindMatchesCount(data) {}
+
+  initPassiveLoading(callbacks) {}
+
+  reportTelemetry(data) {}
+
+  async createL10n() {
+    throw new Error("Not implemented: createL10n");
+  }
+
+  createScripting() {
+    throw new Error("Not implemented: createScripting");
+  }
+
+  updateEditorStates(data) {
+    throw new Error("Not implemented: updateEditorStates");
+  }
+
+  async getNimbusExperimentData() {}
+}
+
+export { BaseExternalServices };