chore(release): prepare 0.6.6

[leehack]

Summary

• bump the package version to 0.6.6
• publish the current release notes in the changelog and recent-releases docs
• update installation snippets to reference ^0.6.6

Testing

• dart format --output=none --set-exit-if-changed .
• dart analyze
• dart test
• ./tool/docs/build_site.sh
• ./tool/docs/validate_links.sh

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.23%. Comparing base (0050bf5) to head (d3e91f9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #86   +/-   ##
=======================================
  Coverage   77.23%   77.23%           
=======================================
  Files          67       67           
  Lines        8338     8338           
=======================================
  Hits         6440     6440           
  Misses       1898     1898           

Flag	Coverage Δ
unittests	77.23% <ø> (ø)

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