Allow scaffold to add a new version for an existing library

[jormundur00]

Summary

• allow scaffold to generate a new metadata/test scaffold for a fresh version of an already supported groupId:artifactId
• keep duplicate exact-version scaffolds blocked unless --force is used
• preserve the correct latest entry when appending versions, including older versions
• format updated index.json files with the repositorys multi-line pretty-print style

Details

• replace the old artifact-root existence check with version-aware checks against the metadata version directory, test directory, and existing index.json entry
• append a new metadata entry to an existing artifact index.json instead of failing just because the artifact root already exists
• recompute latest from the highest metadata version after updates instead of relying on insertion order
• align ScaffoldTask JSON output with the formatting used by the other index update tasks

Testing

• ../../gradlew test --tests org.graalvm.internal.tck.ScaffoldTaskTests --stacktrace

Fixes: #1665