TOML file validator

As we are transiting from old `.conf` to `.toml` for user settings, add a job to run it with `Github Actions` that validate TOML file if it is found in the source tree.

- [ ] Investigate [toml-validator](https://pypi.org/project/toml-validator/) package
- [ ] Add a command to `Makefile` to run toml validation
- [ ] Add a step to run `make validate-toml` in github workflow

Start by investigating the toml-validator package, then inspect the repository's Makefile and GitHub Actions workflow. Add a Makefile command that validates TOML files found in the source tree and invoke it from the workflow; done means the validation runs successfully in CI.

Written by the indexing model from the issue text.