Add support for --assignment-- sections in markdown

[adiprathapa]

Summary

• Parses # --assignment-- sections from challenge markdown with a new assignment getter on the Challenge class
• Renders assignment content in the instructions panel with an <hr> separator
• Adds test fixture using freeCodeCamp challenge (review-recursion) and corresponding tests

Should close #88

Test plan

• npx vitest run passes all tests
• Challenges without --assignment-- render unchanged
• Challenges with --assignment-- display the section below description/instructions

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.33%. Comparing base (7f59946) to head (4948b0c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #148   +/-   ##
=======================================
  Coverage   33.33%   33.33%           
=======================================
  Files          11       11           
  Lines         477      477           
  Branches       92       92           
=======================================
  Hits          159      159           
  Misses        313      313           
  Partials        5        5           

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[benoit74]

Please add an entry in CHANGELOG.md

[benoit74]

LGTM, please squash your commits and I will merge.