Ability to use shell

We'd like to be able to use environment variables in our prepublish command, but this seems to require `shell: true` to be passed to `execa.command()`. Currently, we're using a workaround that sets `shell: true` when a `|` is found in the command. It would be great if we could enable it with something like this:

```json
"prepublish": [
{
"command": "do-something --token $SOME_TOKEN",
"shell": true
},
],
```

Start by locating the prepublish command handling and its use of execa.command(). Check how command configuration is parsed, then add coverage for a prepublish entry with shell enabled and an environment-variable command. Done means the documented configuration runs the command through a shell without relying on the pipe workaround.

Written by the indexing model from the issue text.