Enable the ESLint no-debugger and no-alert rules

The `debugger`-statement would only, potentially, make sense during development and we thus want to prevent it from being accidentally included when landing code.
The `alert`, `confirm`, and `prompt` functions should generally be avoided, with the few intended cases manually allowed.

Please find additional details about the ESLint rules at:
 - https://eslint.org/docs/rules/no-debugger
 - https://eslint.org/docs/rules/no-alert

web/app.js

@@ -1667,6 +1667,7 @@ const PDFViewerApplication = {
           "Warning: The PDF is not fully loaded for printing."
         )
         .then(notReadyMessage => {
+          // eslint-disable-next-line no-alert
           window.alert(notReadyMessage);
         });
       return;

web/chromecom.js

@@ -219,6 +219,7 @@ function requestAccessToLocalFile(fileUrl, overlayManager, callback) {
             "\nSelected: " +
             file.name +
             "\nDo you want to open the selected file?";
+          // eslint-disable-next-line no-alert
           if (!confirm(msg)) {
             this.value = "";
             return;