feat: add rule for 'kill two birds with one stone' idiom

[LarytheLord]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything or linked relevant results below
• I made sure the docs are up to date
• I included tests (or that’s not needed)

Description of changes

Add one new misc rule for the idiom kill two birds with one stone.

• New considerate suggestions:
  • solve two problems at once
  • achieve two goals at once
• Added a regression test in test.js
• Regenerated rules.md (new rule table row)

Validation:

• npm run generate
• node --conditions development test.js

[LarytheLord]

Pushed one more follow-up commit: 7aa458a.\n\nI narrowed the coverage tooling pin to c8 7.3.0 (which uses yargs 15) to avoid the Node 25 ESM/CJS runtime break seen in CI. Local validation still passes with npm test.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2a925cd) to head (7aa458a).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main      #131     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files            3         1      -2     
  Lines         6025       315   -5710     
===========================================
- Hits          6025       315   -5710     

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[stuckvgn]

Leaving a note here since this PR and #130 are closely related — both address issues that are blocking speciesism language pattern support from getting merged.

#130 adds the actual speciesism pattern data (data/en/speciesism.yml) and I just pushed a fix there for the file: '' assertion mismatch in test.js (a Node compat issue pre-existing on main).

The c8/Node 25 fix you've made here would also need to be applied to the #130 branch. If it's easier, these could potentially be combined into one PR. Either way, cross-linking for reviewer visibility — both touch the same CI infrastructure and both are part of the same effort to expand the pattern set.

For what it's worth, this kind of speciesism language guidance is gaining traction: webpack just merged similar rules to their Vale linting config on Feb 25, 2026 (webpack/webpack.js.org#7880). Having it in retext-equality means it propagates automatically to alex and any downstream tool that uses this package.

[LarytheLord]

Thanks for cross-linking this. Agreed these are related efforts.\n\nI’ve kept this PR scoped to the idiom rule + CI compatibility update on this branch; #130 can apply the same c8 pin strategy if that branch still hits the Node 25 runtime mismatch.\n\nIf maintainers prefer, I can also split out the CI pin into a tiny dedicated PR for easier reuse/cherry-pick across both branches.