0.6.0

• swift package --disable-sandbox vercel init also writes the local Swift version to
  .swift-version (unless the file exists): the exact release for a swift.org toolchain (like
  6.4.0), major.minor for Xcode's (like 6.4), none for snapshots. Vercel builds then use the
  Swift you develop with, and new Swift releases are opt-in. swiftly reads the same file.
• vercel-install-swift.sh: .swift-version and SWIFT_VERSION match swift.org releases
  however they are named, so 6.3.0 finds the release named 6.3.
• Toolchain check: no hardcoded Swift versions. For Apple's toolchain it suggests
  swiftly install/swiftly use with its major.minor version (or latest); for a missing or
  mismatched Static Linux SDK it points to swift.org's swift sdk install command instead of
  guessing a URL. An SDK named without a patch version (like swift-6.3-RELEASE_static-linux-0.1.0)
  now matches its toolchain.
• CI tests Swift 6.4 and the latest Swift release, also daily, and installs the matching Static
  Linux SDK with the new scripts/install-static-sdk.sh.
• scripts/test-plugin.sh (replacing scripts/test-vercel-json.sh) tests all of the plugin's
  PackagePlugin-free logic, with the tests in scripts/plugin-tests.