Update CI workflow to test Swift Package Manager

[mattt]

Related to #245

This PR updates the CI workflow (cmake.yml) to add jobs for building yyjson with Swift Package Manager. This ensures that everything works as expected for developers consuming yyjson as a Swift package dependency.

[mattt]

5d19724 adds a job that requires the traits defined by #245. If these changes look good, I'd recommend either merging just 5fb6edc, or cherry-picking these into #245.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.49%. Comparing base (c3fc708) to head (5d19724).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #246   +/-   ##
=======================================
  Coverage   98.49%   98.49%           
=======================================
  Files           2        2           
  Lines        7686     7686           
=======================================
  Hits         7570     7570           
  Misses        116      116           

Flag	Coverage Δ
unittests	98.49% <ø> (ø)

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