Improve metrics APIs to make them more useful.

[JimBobSquarePants]

Prerequisites

• [x] I have written a descriptive pull-request title
• [x] I have verified that there are no overlapping pull-requests open
• [x] I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules :cop:.
• [x] I have provided test coverage for my change (where applicable)

Description

• Adds a method to get kerning offsets to Font
• Updated enumerable return types to indicate that the type is fully enumerated and expose indexers
• Used TryGet..Out pattern where appropriate
• Ensured that all returned glyph metrics take into consideration text attribute options.

@ThomasMiz I'd really like to discuss any further improvements we can add and any issues you have. Your input is really valuable here as it's difficult for us to ensure use cases are covered since our own are quite narrow.

[codecov[bot]]

Codecov Report

Merging #323 (ae2a33f) into main (18401a0) will increase coverage by 0%. The diff coverage is 77%.

@@          Coverage Diff           @@
##            main    #323    +/-   ##
======================================
  Coverage     83%     83%            
======================================
  Files        224     224            
  Lines      12331   12439   +108     
  Branches    1790    1790            
======================================
+ Hits       10306   10424   +118     
+ Misses      1603    1597     -6     
+ Partials     422     418     -4     

Flag	Coverage Δ
unittests	83% <77%> (+<1%)	:arrow_up:

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

Impacted Files	Coverage Δ
src/SixLabors.Fonts/FontMetrics.cs	100% <ø> (ø)
src/SixLabors.Fonts/GlyphBounds.cs	83% <ø> (ø)
...nts/Tables/AdvancedTypographic/GPos/AnchorTable.cs	34% <0%> (-3%)	:arrow_down:
src/SixLabors.Fonts/Tables/Cff/CffBoundsFinder.cs	69% <0%> (-2%)	:arrow_down:
...bors.Fonts/Tables/Cff/TransformingGlyphRenderer.cs	51% <0%> (-3%)	:arrow_down:
src/SixLabors.Fonts/GlyphRendererParameters.cs	59% <38%> (-4%)	:arrow_down:
src/SixLabors.Fonts/Font.cs	53% <52%> (-5%)	:arrow_down:
src/SixLabors.Fonts/StreamFontMetrics.cs	83% <61%> (-4%)	:arrow_down:
src/SixLabors.Fonts/GlyphMetrics.cs	73% <70%> (+22%)	:arrow_up:
...Tables/AdvancedTypographic/Shapers/HangulShaper.cs	83% <75%> (+<1%)	:arrow_up:
... and 14 more

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