Refactor to `configService` to read & write `Config`

`configService` can save and load `Config` using a strategy of `configEncoder`
and `configDecoder`. This pave the path for swapping out `tomlConfigEncoder`
and `tomlConfigDecoder` with `yamlConfigEncoder` and `yamlConfigDecoder`.

`Configs` also has been renamed to `Config`.

/cc @mislav