housekeeping: add .net core 3.0 support

[glennawatson]

• Now has .net core 3 as a available status.

[codecov]

Codecov Report

Merging #349 into master will increase coverage by 0.06%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #349      +/-   ##
==========================================
+ Coverage   70.59%   70.66%   +0.06%     
==========================================
  Files          65       65              
  Lines        3551     3552       +1     
  Branches      317      317              
==========================================
+ Hits         2507     2510       +3     
+ Misses       1001     1000       -1     
+ Partials       43       42       -1

Impacted Files	Coverage Δ
src/Splat/Platforms/net4/Maths/RectExtensions.cs	0% <ø> (ø)
...plat/Platforms/net4/Colors/SplatColorExtensions.cs	0% <ø> (ø)
...Splat/Platforms/net4/Bitmaps/BitmapSourceBitmap.cs	0% <ø> (ø)
src/Splat/Platforms/net4/Bitmaps/BitmapMixins.cs	0% <ø> (ø)
src/Splat/Platforms/net4/Maths/SizeExtensions.cs	0% <ø> (ø)
src/Splat/Platforms/net4/Colors/ColorExtensions.cs	0% <ø> (ø)
...lat/Platforms/net4/Bitmaps/PlatformBitmapLoader.cs	0% <ø> (ø)
src/Splat/Platforms/net4/Maths/PointExtensions.cs	0% <ø> (ø)
src/Splat/TargetFrameworkExtensions.cs	93.54% <0%> (+0.21%)	⬆️
src/Splat/MemoizingMRUCache.cs	58.62% <0%> (+2.29%)	⬆️

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