Support loading global settings across all projects

[jonhoo]

There are some rls settings that I would like to be enabled for all my projects, and not have to remember to re-set for every new project. In particular, things like build_on_save, clippy_preference, and show_hover_context seem reasonable to want to share across projects. I'd be neat if rls could read a file like $XDG_CONFIG_HOME/rls/settings.json at start-up, and then merge that with per-project settings if the client sends any!