Modernize toolchain and improve test coverage

[jasonjoh]

Summary

Upgrades the project to modern ESM tooling, Node 24, and current dependency versions. Removes unnecessary dependencies, improves test mocking, and fixes a case-sensitivity bug in file exclusion matching.

Changes

Toolchain modernization

• Migrate from CommonJS to native ESM ( "type": "module" ,  .js  import extensions,  nodenext  module resolution)
• Upgrade Node runtime from  node20  to  node24  in  action.yml 
• Replace  @vercel/ncc  bundler with Rollup
• Replace  .eslintrc.json  with ESLint flat config ( eslint.config.js )
• Upgrade major dependencies: TypeScript 6, ESLint 10, Jest 30,  @actions/core  3,  @actions/github  9, minimatch 10
• Update  tsconfig.json  to target ES2022 with  nodenext  resolution
• Add copyright headers to all source files
• Bump version to 1.0.15

Test improvements

• Remove  fetch-mock ,  node-fetch , and  @types/node-fetch  dependencies
• Replace  fetch-mock  with  jest.fn()  + native  Response  objects injected via Octokit's  request: { fetch }  option
• Fix  jest.config.js  to use proper ESM  export default  instead of  module.exports 
• Add 6 new tests covering previously untested paths:
  • checkFilesForCrlf  with exclusion patterns
  • isFileExcluded  with empty array (default fallback)
  • checkFilesForCrlf  with empty file list
  • generatePrComment  with a single file
• Bare  \r  (old Mac-style) not flagged as CRLF
• Case-insensitive file extension matching

Bug fix

• File exclusion patterns are now case-insensitive ( { nocase: true }  passed to  minimatch ), so patterns like  /.png  also match  .PNG ,  .Png , etc.

Documentation

• Add .github/copilot-instructions.md  for AI-assisted development context

Testing

All 14 tests pass ( npm test ).