Swift6 prep

[heckj]

• updated package to use Swift 5.8 minimum
• annotated methods and access that work with CGColorSpace and CGColor with @mainactor to constrain them for concurrency
• updated general format flow to Swift 5.8 patterns (swiftformat .)

[codecov]

Codecov Report

Attention: Patch coverage is 78.70968% with 33 lines in your changes are missing coverage. Please review.

Project coverage is 90.45%. Comparing base (ae61e35) to head (7f88458).

Files	Patch %	Lines
Sources/SwiftVizScale/ContinuousScale.swift	66.66%	11 Missing ⚠️
Sources/SwiftVizScale/SequentialScale.swift	0.00%	4 Missing ⚠️
Sources/SwiftVizScale/Color/LCH.swift	40.00%	3 Missing ⚠️
Sources/SwiftVizScale/DomainDataTransform.swift	0.00%	3 Missing ⚠️
...zScale/Interpolators/ComputedRGBInterpolator.swift	0.00%	3 Missing ⚠️
Sources/SwiftVizScale/ContinuousScaleType.swift	80.00%	2 Missing ⚠️
Sources/SwiftVizScale/DiscreteScale.swift	0.00%	2 Missing ⚠️
Sources/SwiftVizScale/BandScale.swift	92.30%	1 Missing ⚠️
Sources/SwiftVizScale/Histogram/Histogram.swift	94.44%	1 Missing ⚠️
...Scale/Interpolators/IndexedColorInterpolator.swift	75.00%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
+ Coverage   90.41%   90.45%   +0.03%     
==========================================
  Files          40       40              
  Lines        3653     3634      -19     
==========================================
- Hits         3303     3287      -16     
+ Misses        350      347       -3     

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