Source generator - Attribute for named style

[sveinungf]

Implements #44

[codecov]

Codecov Report

Attention: Patch coverage is 91.90283% with 20 lines in your changes missing coverage. Please review.

Project coverage is 89.84%. Comparing base (956b7fb) to head (bafeaeb).
Report is 37 commits behind head on main.

Files with missing lines	Patch %	Lines
...rceGenerator/Extensions/AttributeDataExtensions.cs	70.00%	10 Missing and 8 partials ⚠️
...readCheetah/SourceGeneration/CellStyleAttribute.cs	0.00%	1 Missing ⚠️
SpreadCheetah/Spreadsheet.cs	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
+ Coverage   89.73%   89.84%   +0.10%     
==========================================
  Files         152      156       +4     
  Lines        4989     5131     +142     
  Branches      941      955      +14     
==========================================
+ Hits         4477     4610     +133     
- Misses        208      215       +7     
- Partials      304      306       +2     

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