Add ability for markdown file browser to display json files

[brimock]

Why are these changes needed?

Enable the MarkdownFileBrowser to display '.json' files, such as a package.json file of a react package.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[brimock]

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[afourney]

This will work as-is, and I'll approve it.

Note however that the next version of MarkItDown will support json files, and so we may revisit this.

[codecov]

Codecov Report

Attention: Patch coverage is 18.51852% with 22 lines in your changes missing coverage. Please review.

Project coverage is 78.62%. Comparing base (48242f5) to head (c99fd9d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...n_ext/agents/file_surfer/_markdown_file_browser.py	18.51%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5463      +/-   ##
==========================================
+ Coverage   71.91%   78.62%   +6.70%     
==========================================
  Files         236      167      -69     
  Lines       11922    10044    -1878     
  Branches      238        0     -238     
==========================================
- Hits         8574     7897     -677     
+ Misses       3163     2147    -1016     
+ Partials      185        0     -185     

Flag	Coverage Δ
unittests	78.62% <18.51%> (+6.70%)	⬆️

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