feat: Add update and version commands

[liamg]

Summary

• Add update command that self-updates the CLI binary to the latest GitHub release
• Add version command that prints the current version

Details

The update command:

• Fetches the latest release from infracost/cli via the GitHub API
• Compares it against the current build version using semver (dev builds always update)
• Downloads the platform-specific asset ({os}_{arch}.tar.gz / .zip) and extracts the binary
• Atomically replaces the running binary via temp file + rename
• Authenticates using GH_TOKEN, GITHUB_TOKEN, or gh auth token if available; falls back to unauthenticated access otherwise
• Looks for infracost-preview in the asset bundle, and falls back to infracost if not found. This will help when we eventually remove the -preview suffix.
• Uses GH_TOKEN, GITHUB_TOKEN, or gh auth token to authenticate with GitHub to download the asset. Downloads anonymously if this fails.

[liamcervante]

Update the README as well?