chore: prepare release 0.4.2

[waynexia]

For #94, which

• Adds a new feature gate ser that brings serialization support for the AST
• Adds new deps serde and serde_json under that feature gate

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.97%. Comparing base (7691247) to head (60dde86).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #96   +/-   ##
=======================================
  Coverage   98.97%   98.97%           
=======================================
  Files          14       14           
  Lines        6467     6467           
=======================================
  Hits         6401     6401           
  Misses         66       66           

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