Enhancements to modernize branch

[alercunha]

Summary

• Add modern Python packaging with pyproject.toml (replaces setup.py)
• Add multi-stage Dockerfile for building and testing
• Add comprehensive CI workflow with build, test, and pylint
• Fix error messages for brand/label character limits
• Fix exit codes in lv2_validate_mod

Changes

Packaging

• Replace setup.py with pyproject.toml
• Add scripts/build.sh and scripts/test.sh for consistent build/test workflow

CI

• Replace separate pylint.yml with unified build.yml workflow
• Pylint runs as part of test.sh (no duplicate virtualenv creation)

Bug Fixes

• Fix brand error message: "11 characters" → "16 characters"
• Fix label error message: "16 characters" → "24 characters"
• Add proper exit 1 codes to lv2_validate_mod error conditions

Docker

• Multi-stage build (builder → tester → artifacts)
• Clean test environment validates the built packages

Test plan

• CI workflow runs automatically on push/PR
• Docker build validates end-to-end: docker build -t lilvlib-build .