Fix CommonJS / TypeScript types; clean up ESLint configurations

[joshkel]

Specific changes:

• Clean up ESLint configurations: ESLint checks eslint.config.js before checking eslint.config.mjs, so eslint.config.mjs had no effect. Calling ESLint's configuration file .mjs ensures that it's always recognized as ESM, regardless of package.json.
• Delete old .eslintrc. It has no effect in ESLint 9.
• Delete "type": "module". As I understand it, hapi-auth-jwt2 itself is still CommonJS. "type": "module" was added to make ESLint 9.x configurations happier, but preceding commits should address that.
• Add "Are the Types Wrong" to help catch future issues with TypeScript types, such as the one fixed here.

Fixes #511

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0855867) to head (488f78b).
⚠️ Report is 32 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #545   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          155       155           
  Branches        45        45           
=========================================
  Hits           155       155           

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