Implement Hangul Shaper

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

Fixes #213

Adds the Hangul Shaper to the shaping collection.

TODO

• Complete shaper
• Add tests (@brianpopow if you have any time available it'd be great if you could help out there.)

[codecov]

Codecov Report

Merging #262 (6a0947e) into main (2832534) will increase coverage by 0%.
The diff coverage is 86%.

@@         Coverage Diff          @@
##           main   #262    +/-   ##
====================================
  Coverage    83%    83%            
====================================
  Files       187    188     +1     
  Lines      9517   9848   +331     
  Branches   1499   1587    +88     
====================================
+ Hits       7939   8222   +283     
- Misses     1231   1260    +29     
- Partials    347    366    +19     

Flag	Coverage Δ
unittests	83% <86%> (+<1%)	⬆️

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

Impacted Files	Coverage Δ
...Tables/AdvancedTypographic/Shapers/ArabicShaper.cs	89% <ø> (-1%)	⬇️
...abors.Fonts/Tables/AdvancedTypographic/TagEntry.cs	100% <ø> (ø)
src/SixLabors.Fonts/GlyphMetrics.cs	60% <46%> (-2%)	⬇️
...es/AdvancedTypographic/GSub/LookupType4SubTable.cs	74% <50%> (ø)
src/SixLabors.Fonts/TextLayout.cs	85% <62%> (-1%)	⬇️
...es/AdvancedTypographic/GPos/LookupType3SubTable.cs	58% <66%> (+1%)	⬆️
src/SixLabors.Fonts/GlyphShapingData.cs	93% <83%> (+7%)	⬆️
...Tables/AdvancedTypographic/Shapers/HangulShaper.cs	83% <83%> (ø)
src/SixLabors.Fonts/GlyphPositioningCollection.cs	90% <86%> (-2%)	⬇️
src/SixLabors.Fonts/GlyphSubstitutionCollection.cs	89% <87%> (-9%)	⬇️
... and 26 more

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[JimBobSquarePants]

@brianpopow I'm afraid this turned into a bit of a beast. I had to refactor the shaping collections to correctly handle glyph decomposition.

On a happy note though I've got the shaper working really well now and I was able to remove a whole bunch of indexers.

I have a bunch of test codepoint sequences locally that I can create tests from so don't worry about them. I'll likely do them tomorrow night.