Add category PURCHASES

[mbrt]

This is new and wasn't originally supported.

Close #451.

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.60%. Comparing base (4690854) to head (42e1403).

Files with missing lines	Patch %	Lines
internal/engine/export/xml/consts.go	0.00%	2 Missing ⚠️
internal/engine/gmail/consts.go	0.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (57.14%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #453      +/-   ##
==========================================
+ Coverage   37.56%   37.60%   +0.03%     
==========================================
  Files          54       54              
  Lines        3663     3670       +7     
==========================================
+ Hits         1376     1380       +4     
- Misses       2196     2199       +3     
  Partials       91       91              

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