Added derive for arbitrary

[LucaCappelletti94]

Adds Arbitrary derives to all AST structs and enums for property-based testing and fuzzing.

Changes

• Added arbitrary as optional dependency with feature gating
• Derived Arbitrary on all public AST types in ast
• Added Arbitrary to core types (Token, Span, Location, etc.)
• Excluded display helpers and iterators

Note: BigDecimal (used in Value::Number) lacks Arbitrary support. PR submitted: akubera/bigdecimal-rs#163

[github-actions]

Thank you for your contribution. Unfortunately, this pull request is stale because it has been open 60 days with no activity. Please remove the stale label or comment or this will be closed in 7 days.

[LucaCappelletti94]

This type of GitHub Actions is not positive IMHO. This PR is blocked by another PR uphill, and closing it does not make the problem go away.

[LucaCappelletti94]

Will need rebasing, but finally akubera/bigdecimal-rs#163 has been merged and hopefully a new version of bigdecimal will be published soonish.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.15%. Comparing base (c70b5b1) to head (82d0288).

Files with missing lines	Patch %	Lines
src/ast/value.rs	83.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2193   +/-   ##
=======================================
  Coverage   81.14%   81.15%           
=======================================
  Files          42       42           
  Lines       33739    33751   +12     
  Branches    33739    33751   +12     
=======================================
+ Hits        27379    27389   +10     
  Misses       2797     2797           
- Partials     3563     3565    +2     

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