internal : redesign rust-analyzer::config
This PR aims to cover the infrastructural requirements for the `rust-analyzer.toml` ( #13529 ) issue. This means, that
1. We no longer have a single config base. The once single `ConfigData` has been divided into 4 : A tree of `.ratoml` files, a set of configs coming from the client ( this is what was called before the `CrateData` except that now values do not default to anything when they are not defined) , a set of configs that will reflect what the contents of a `ratoml` file defined in user's config directory ( e.g `~/.config/rust-analyzer/.rust-analyzer.toml` and finally a tree root that is populated by default values only.
2. Configs have also been divided into 3 different blocks : `global` , `local` , `client`. The current status of a config may change until #13529 got merged.
Once again many thanks to `@cormacrelf` for doing all the serde work.
1179c3ee83