feat: Add MDX language support

[GeekCornerGH]

Proper implementaion of #1253

cf00d5b adds MDX to the map of known languages
7e3cb34 routes any file ending in MDX to the WritingDocsCategory instead of unknown

Development was done against Wakapi, but should work on the official Wakatime instance without any change

[gandarez]

let's hold this PR a little as we're implementing a new dynamic way to generate language maps that may affect and conflict with this one.

#1256

[GeekCornerGH]

I see unit tests are failing, but they were failing on a fresh clone too for me, did I broke something by mistake?

[alanhamlett]

@GeekCornerGH it's because @gandarez just refactored our Chroma language detection to auto-generate language mappings yesterday.

[GeekCornerGH]

@alanhamlett @gandarez apologies for the notification spam, Git decided not to include the latest merge commit in my local branch for no reason. Should be good now.

[gandarez]

Let me rethink the strategy for non-mapped languages in Chroma

[gandarez]

Just found a pretty solution that enables adding missing languages via comment.

#1258

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.17%. Comparing base (a1d8211) to head (f5f028b).
⚠️ Report is 2 commits behind head on develop.

@@           Coverage Diff            @@
##           develop    #1257   +/-   ##
========================================
  Coverage    53.17%   53.17%           
========================================
  Files          394      394           
  Lines        11496    11496           
========================================
  Hits          6113     6113           
  Misses        4841     4841           
  Partials       542      542           

Flag	Coverage Δ
unittests	53.17% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pkg/heartbeat/category.go	94.16% <100.00%> (ø)

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