Rename FeatureTags to match standard

[JimBobSquarePants]

Prerequisites

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

Description

This pull request refactors the handling of OpenType feature tags throughout the codebase. The main change is renaming the FeatureTags enum to KnownFeatureTags and updating all references to use the new name. This improves clarity and consistency, as well as future-proofs the code for distinguishing between known and custom feature tags. The changes affect both the main library and related tests.

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82%. Comparing base (6011281) to head (e50629b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...Tables/AdvancedTypographic/Shapers/HangulShaper.cs	75%	1 Missing ⚠️
...Tables/AdvancedTypographic/Shapers/HebrewShaper.cs	0%	1 Missing ⚠️
...ables/AdvancedTypographic/Shapers/MyanmarShaper.cs	0%	1 Missing ⚠️
src/SixLabors.Fonts/TextLayout.cs	50%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #521   +/-   ##
=====================================
  Coverage     82%     82%           
=====================================
  Files        305     305           
  Lines      21534   21534           
  Branches    3165    3165           
=====================================
  Hits       17818   17818           
  Misses      2901    2901           
  Partials     815     815           

Flag	Coverage Δ
unittests	82% <84%> (ø)

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