Migrate to ESLint flat config

Flat config is the new config system used by ESLint 9.
To make the migration easier, they also added
flat config support to ESLint 8.

This commit migrates the various ESLint configs in the repository to use
the new system, **without** upgrading to ESLint 9 yet.

gulpfile.mjs

@@ -12,7 +12,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* eslint-env node */
 
 import {
   babelPluginPDFJSPreprocessor,
@@ -1956,8 +1955,6 @@ gulp.task("lint", function (done) {
   // Ensure that we lint the Firefox specific *.jsm files too.
   const esLintOptions = [
     "node_modules/eslint/bin/eslint",
-    "--ext",
-    ".js,.jsm,.mjs,.json",
     ".",
     "--report-unused-disable-directives",
   ];