Remove the abstract BaseViewer-class

After the changes in PR 14112 the `PDFViewer`-class is now "identical" to the `BaseViewer`-class and the `PDFSinglePageViewer`-class is just a very thin wrapper around the `BaseViewer`-class.
Hence we can rename these files, and also remove the abstract `BaseViewer`-class, which helps reduce some unnecessary "closures" in the *built* viewer.

*Please note:* These changes are made in two separate commits, to allow GitHub to preserve `blame` for the affected files.

web/pdf_single_page_viewer.js

@@ -0,0 +1,37 @@
+/* Copyright 2014 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.
+ */
+
+import { ScrollMode, SpreadMode } from "./ui_utils.js";
+import { PDFViewer } from "./base_viewer.js";
+
+class PDFSinglePageViewer extends PDFViewer {
+  _resetView() {
+    super._resetView();
+    this._scrollMode = ScrollMode.PAGE;
+    this._spreadMode = SpreadMode.NONE;
+  }
+
+  // eslint-disable-next-line accessor-pairs
+  set scrollMode(mode) {}
+
+  _updateScrollMode() {}
+
+  // eslint-disable-next-line accessor-pairs
+  set spreadMode(mode) {}
+
+  _updateSpreadMode() {}
+}
+
+export { PDFSinglePageViewer };