Add `vera version` CLI command

There is currently no way to check the installed Vera version from the CLI. Add:

```
$ vera version
vera 0.0.101
```

Useful for bug reports, CI scripts, and agent tooling that needs to check compatibility. The version is already available from `vera.__version__` — this is a thin CLI wrapper.

Also consider whether `vera --version` / `vera -V` should work as aliases.