feat: support configurable version storage backends

[oflynned]

Summary

• add a version_storage input with version-properties as the compatibility default
• add gradle-properties support for reading and writing version metadata in gradle.properties
• preserve unrelated gradle.properties keys while updating version keys
• stage the selected version storage file when creating the release commit
• document supported storage backends, expected keys, and file formats

Fixes #135

Tests

• npm run lint
• npm run typecheck
• npm test -- --runInBand
• npm run test:e2e