Configure ESLint with flat config and StandardJS

[abhisek7154]

Fix for issue #4288

Migrated the project to ESLint's new flat config system (eslint.config.cjs), required as of ESLint v9. StandardJS was used as the base style guide, with a few adjustments to match Less.js's existing code style and avoid widespread formatting changes. Also added a lint script to package.json.

The project previously had no working ESLint configuration compatible with ESLint v9. This update brings the codebase into alignment with the latest ESLint standards and enables consistent, enforceable linting going forward.

Checklist:

• Documentation (N/A)
• Added/updated unit tests (N/A)
• [ x ] Code complete

This PR is designed to introduce linting with minimal disruption to the existing codebase. Further cleanup can be incremental once the config is in place.

[matthew-dean]

Thanks for taking a look. Note that there should not be both an NPM lock file and a PNPM lock file. PNPM should be the manager IIRC.