Some refactoring to `config.rs` with the aim to:
* Increase type safety
* Reduce duplication
* Leverage `std::result::Result` & `std::path::PathBuf` APIs
To achieve these goals, the following was done:
* `get_config_file` propagates an `FF2MpvError` if the config file doesn't exist/is invalid
* Always type check `unix` & `windows` code by using `cfg!` expressions over
`#[cfg()]` attributes. This also removed some duplicated lines of code
* Move `#[serde(default)]` to `Config` struct instead of every struct
member: https://serde.rs/container-attrs.html#default
