[api-minor] Remove the Webpack-only npm dependencies from pdfjs-dist (PR 11418 follow-up)

Currently *all* users of `pdfjs-dist` are forced to install the `webpack` and `worker-loader` packages, despite the fact that they are *only* relevant if the `webpack.js` file is being used (with a custom Webpack build).
This really doesn't seem great, especially since those packages are the only remaining dependencies in the `pdfjs-dist` library, and it thus seem more reasonable overall that Webpack users handle those dependencies themselves.

To prevent unnecessarily cryptic runtime failures, when people update to newer `pdfjs-dist` versions, the `webpack.js` file was updated to explicitly check for the existence of the `worker-loader` package and error otherwise.
Furthermore, note that `webpack` was only listed as a dependency because of the `worker-loader` package itself (see issue 9248).

Obviously these changes may not be seen as great by Webpack users who rely on `pdfjs-dist`, since it forces them to handle the dependencies themselves, however it should improve things considerably for "general" users of `pdfjs-dist` by not burdening them with unnecessary dependencies.
These sort of changes are also in line with other recent changes, see PR 11418, which removed built-in fake worker loader code for specific JS builders/bundlers/frameworks. This work was prompted not only by a desire to simplify/clean-up old code, but also to lessen future support burden since the PDF.js contributors cannot be assumed to be experts in various JS bundlers.

external/dist/webpack.js

@@ -12,8 +12,17 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 "use strict";
 
+try {
+  require.resolve("worker-loader");
+} catch (ex) {
+  throw new Error(
+    "Cannot find the `worker-loader` package, please make sure that it's correctly installed."
+  );
+}
+
 var pdfjs = require("./build/pdf.js");
 var PdfjsWorker = require("worker-loader!./build/pdf.worker.js");