Restore support for package managers other than `npm`

In https://github.com/arethetypeswrong/arethetypeswrong.github.io/pull/180, support for `pnpm`/`yarn` was removed, for reasons that aren't super clear (I guess there was a bug with the support? https://github.com/arethetypeswrong/arethetypeswrong.github.io/issues/179).

I'd propose support for packing with `pnpm`/`yarn` be restored. If not via automatic detection, at least to the point of being able to specify a way to use `attw` in CI neatly. For example, while `attw $(npm pack)` is documented, and works fine, `attw $(pnpm pack)` fails, with:

```plaintext
error while checking file:
ENOENT: no such file or directory, open '📦'
```

Start by reproducing the documented `attw $(npm pack)` flow and the failing `attw $(pnpm pack)` command, then inspect the `attw` CLI entry point and its package-file argument handling. Done means pnpm and yarn packing are supported, or a clear CI invocation is available, without treating pnpm's `📦` output as a filename.

Written by the indexing model from the issue text.