[meta] Update flatConfig example to include typescript config

[intellix]

Was having an issue for a few hours trying to get import/no-unresolved not picking up relative imports that don't include the extension like so:

import { add } from './utils';

After asking inside Discord, kkapp helped me by pointing out that I may need to configure the resolver to look for "ts" extensions. After adding it I went on a crusade to figure out how I had managed to miss that when migrating to flat configs.

The example given for non-flat configs tells you to include the typescript config, but the flatConfig variant does not, so for an out-of-the-box experience when migrating, it can cause a headache.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.61%. Comparing base (fa36d49) to head (bbc61d6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3138      +/-   ##
==========================================
+ Coverage   95.47%   95.61%   +0.13%     
==========================================
  Files          83       83              
  Lines        3628     3629       +1     
  Branches     1281     1282       +1     
==========================================
+ Hits         3464     3470       +6     
+ Misses        164      159       -5     

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[ljharb]

Thanks!