Add CLI version flag

  ## Goal

  Add a CLI flag that lets users check which version of `treels` they are running.

  ## Context

  The current unreleased main version is expected to be `v1.3.0`. Users should be able to check this directly from the CLI.

  ## Acceptance Criteria

  - Add `-v, --version`
  - Running `treels --version` prints:

    ```text
    treels v1.3.0