Dedicated configuration

Use a separate file for config instead of `sfdx-project` `plugins`.

- Provide JS and JVM support for reading the config so clients can use it when server is off.
- Allow clients to include their own section to be loaded with JS API (similar to `plugins` entries).
- Just get the named section/property as json string and return, let client do validation and parsing.
- Library options: [`circe-config`](https://github.com/circe/circe-config) (using other libs) | [`sconfig`](https://github.com/ekrich/sconfig) (jvm/js port of lightbend)