Add FontCollection.AddSystemFontCollection

[JimBobSquarePants]

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• I have written a descriptive pull-request title
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Description

Implements FontCollection.AddSystemFontCollection() extension and normalizes the process of adding metrics to the underlying collection.

[codecov]

Codecov Report

Merging #195 (5899252) into master (97e3ed6) will increase coverage by 0.09%.
The diff coverage is 91.30%.

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
+ Coverage   86.91%   87.00%   +0.09%     
==========================================
  Files          98       99       +1     
  Lines        4654     4656       +2     
  Branches      758      757       -1     
==========================================
+ Hits         4045     4051       +6     
+ Misses        484      482       -2     
+ Partials      125      123       -2     

Flag	Coverage Δ
unittests	87.00% <91.30%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/SixLabors.Fonts/FontCollection.cs	87.50% <87.50%> (+3.62%)	⬆️
src/SixLabors.Fonts/FontCollectionExtensions.cs	100.00% <100.00%> (ø)
src/SixLabors.Fonts/SystemFontCollection.cs	95.83% <100.00%> (+0.37%)	⬆️

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[tocsoft]

Suggested change

	public static FontCollection AddSystemFontCollection(this FontCollection collection)
	public static FontCollection AddSystemFonts(this FontCollection collection)

reads better and prevents users needing to understand that we have a separate collection for system fonts laying around (that feels like implementation detail to me)

[JimBobSquarePants]

Yeah that’s better! I chose the original name to match our use of Collection suffix elsewhere but that actually makes things for confusing since thats for ttc files.

[JimBobSquarePants]

Done