Add self-update command

[Fafoooo]

Adds an arbella update command for updating Arbella itself through npm.

Changes:

• Register arbella update in the CLI.
• Add --version and --dry-run options.
• Document the command in the README.
• Add unit coverage for command registration and update behavior.
• Bump package version to 0.1.3 for the next npm release.

Validation:

• npm run typecheck
• npm test
• npm pack --dry-run
• git diff --check