Add custom subtypes function to avoid deprecation warnings

[dgleich]

Fixes #235

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 69.73%. Comparing base (0bd5d85) to head (43a7f98).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/libmagickwand.jl	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #236      +/-   ##
==========================================
+ Coverage   68.43%   69.73%   +1.30%     
==========================================
  Files           3        3              
  Lines         434      456      +22     
==========================================
+ Hits          297      318      +21     
- Misses        137      138       +1     

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[ViralBShah]

@timholy Would be good if you can review.