Add support for grayscale modes

[regexident]

The current implementation of final func grayscaled() -> DynamicColor uses the HSL lightness as as measurement of lightness.

While this is computationally convenient it does not provide a good match for the human perception of lightness.

Relative luminance (the Y in XYZ) provides a much more accurate measure.

This PR adds support for specifying which color space (i.e. xyz, hsl, rgb, hsv) to be used for grayscaling, defaulting to .lightness (aka HSL) for the sake of not introducing a breaking-change.

⚠️ Depends on https://github.com/yannickl/DynamicColor/pull/56

[codecov[bot]]

Codecov Report

Merging #55 into master will increase coverage by 0.19%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   93.04%   93.24%   +0.19%     
==========================================
  Files          14       14              
  Lines         374      385      +11     
==========================================
+ Hits          348      359      +11     
  Misses         26       26

Flag	Coverage Δ
#ios	93.24% <100%> (+0.19%)	:arrow_up:
#osx	5.97% <0%> (-0.18%)	:arrow_down:

Impacted Files	Coverage Δ
Sources/Utils.swift	100% <ø> (ø)	:arrow_up:
Sources/DynamicColor+RGBA.swift	100% <100%> (ø)	:arrow_up:
Sources/DynamicColor+Deriving.swift	100% <100%> (ø)	:arrow_up:

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[yannickl]

Looks good to me, thanks!