feat: report error decreases in --check-files flag

[frankieyan]

Summary

When you fix React Compiler errors, it's nice to know your fixes actually worked. Right now, the --check-files flag only tells you when things get worse (error counts go up), but stays silent when things get better.

This PR updates the check to also report when error counts decrease. So if you fix a few errors and run the check, you'll see a message like:

🎉 React Compiler errors have decreased in:
  • src/MyComponent.tsx: -3

Fixes #35

Test plan

• Build the project and test against the sample fixtures:

   npm run build

• Create a baseline with the bad fixtures

   cd src/__fixtures__/sample-project
   node ../../../dist/index.mjs --overwrite

• A .react-compiler.rec.json is added with bad-component.ts and bad-hook.ts

Test decreases:

• Fix the bad-hook.ts violations by removing all ref access
• Run check-files:

   node ../../../dist/index.mjs --check-files src/bad-hook.ts

• You should see the decrease message for any files that previously had errors recorded.

Test increases:

• Introduce an error in good-hook.ts by running one of the hooks conditionally
• Run check-files:

   node ../../../dist/index.mjs --check-files src/good-hook.ts

• Should still exit with error when errors increase.

[doistbot-app]

This PR successfully implements reporting for error decreases and handles deleted files in records. The logic for calculating changes looks correct. I have a few suggestions to align the implementation with the updated README (file validation) and improve the user experience (reporting order and redundant logs).

[doistbot-app]

The feature to report error decreases is a great addition for visibility. However, there are two critical issues that need to be addressed to ensure the code functions correctly: checkErrorChanges needs to return the records object for its caller, and the definition of newErrorCount appears to be missing in getErrorChanges, which would cause a runtime error.