chore: refactor tooling and build configuration

[KristjanESPERANTO]

This PR updates the project's linting, build configuration, and package distribution.

Linting & Code Quality

• Migrate to new ESLint config patterns
• Replace stylelint with @eslint/css for unified linting
• Add GitHub Actions workflow for automated linting
• Enable some recommended rules via @eslint/js
  • Fix linting issues in source and demo files

Build Configuration

• Remove unused Rollup plugins (node-resolve, commonjs)
  • Only external dependency is leaflet, no module resolution needed
• Use modern JSON imports with import attributes
• Add version banner to all builds (ESM, UMD, minified)

Package Distribution

• Add exports field for modern module resolution
• Add files field to explicitly list published content
• Add sideEffects field to enable tree-shaking for ESM builds
• Remove redundant unpkg/jsdelivr fields (they fallback to main)
• Remove .npmignore (superseded by files field)

Impact

• Smaller npm package (only ships dist/, not src/ or demo/)
• Better tree-shaking for modern bundlers
• Cleaner, more maintainable configuration

---

Happy new year! 🎆

[domoritz]

Thank you