feat: bundle in CJS and ESM formats

[Belco90]

Checks

• I have read the contributing guidelines.

Changes

• Set up a bundler (tsdown)
• Update package.json fields for CJS and ESM exposed files
• Set up publint for linting the published bundle
• Update type to "module"
• Move CJS code to ESM
• Rename lib/ to src/
• Adjust types (mirroring eslint-plugin-vitest and using eslint exported types)

Context

Closes #900

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (9c78006) to head (287f839).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1130      +/-   ##
==========================================
- Coverage   97.48%   97.47%   -0.01%     
==========================================
  Files          49       49              
  Lines        5997     5974      -23     
  Branches     1518     1518              
==========================================
- Hits         5846     5823      -23     
  Misses        149      149              
  Partials        2        2              

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