Split library and cli into different modules

[ghokun]

Closes #71.

• Adds Gradle modules :lib (java-library, interface-only API in dev.gokhun.convert.lib) and :cli (application + GraalVM native, dev.gokhun.convert.cli).
• Lib impl stays package-private (ConversionUtil/FileType, DefaultConverter/Options); exposed only via Converter/ConverterOptions static factories.
• Binary name unchanged (convert) via applicationName/archivesName/imageName + hardcoded archive name.
• Moves unit tests, fixtures, e2e and native-image metadata into owning module.
• Updates CI workflows, README and build.sh to :cli: tasks and cli/build paths.

Verified: ./gradlew check, :cli:nativeTest, :cli:nativeCompile, :cli:e2e, :cli:generatePackage.