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;