Consider adding rust-version to Cargo.toml

I have a project that has a MSRV of 1.81. I recently did a dependency upgrade, which pulled in the newest argh (and dependencies). This fails:

```
error[E0658]: use of unstable library feature 'iter_repeat_n'
--> /Users/shep/.cargo/registry/src/index.crates.io-6f17d22bba15001f/argh_shared-0.1.14/src/lib.rs:208:21
|
208 | line.extend(std::iter::repeat_n(' ', num_spaces));
| ^^^^^^^^^^^^^^^^^^^
|
```

Obviously it's too late to go back and change 0.1.13 / 0.1.14 😄, but it'd be good to add it *now* so that the next person in the same situation can be helped by Cargo's MSRV-aware resolver.