feat: add # char, nth occurrence of dow in the month

[janrnc]

Partially covers #20

[codecov]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 93.78%. Comparing base (76b7bf0) to head (86ce018).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/parser/dow.go	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
+ Coverage   93.69%   93.78%   +0.09%     
==========================================
  Files          17       17              
  Lines         920      950      +30     
==========================================
+ Hits          862      891      +29     
- Misses         45       46       +1     
  Partials       13       13              

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