CI: add slash dispatch workflow to trigger pre-commit checks with comment
This would be an on-demand bot to run pre-commit checks on a PR, which can be triggered by commenting

```
@github-actions pre-commit
```

on a pull request (see [here](https://github.com/nbQA-dev/nbQA/pull/518) for a demo).

Use case: if a PR is submitted and is good-to-go except for some linting error, we can just comment `/pre-commit-run` and have the bot fixup the errors.