Fix #316 include open type font(otf) in system font collection.

[nakamura2000]

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Description

Fix #316
Add otf extension to system font collection allowed extension.
This test needs installing font to system font directory so I couldn't provide test case in unit test.

1. install otf font in os system font directory. (e.x Fonts/tests/SixLabors.Fonts.Tests/Fonts/NotoSansKR-Regular.otf)

2. list font by this code.

    [Fact]
    public void SystemFonts_HasOTF()
    {
        IEnumerable<FontFamily> matched = SystemFonts.Collection.Families.Where(x => x.Name == "Noto Sans KR");
        Assert.True(matched.Count() != 0);
    }
   

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[codecov]

Codecov Report

Merging #317 (17cf049) into main (b97f40e) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff          @@
##            main    #317   +/-   ##
=====================================
  Coverage     83%     83%           
=====================================
  Files        224     224           
  Lines      12299   12300    +1     
  Branches    1788    1788           
=====================================
+ Hits       10279   10280    +1     
  Misses      1600    1600           
  Partials     420     420           

Flag	Coverage Δ
unittests	83% <100%> (+<1%)	⬆️

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Impacted Files	Coverage Δ
src/SixLabors.Fonts/SystemFontCollection.cs	53% <100%> (+<1%)	⬆️

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