support Rust LSP server auto-detect some useful root path based on location of Cargo.toml

Summary:
now for me rls works for some imple project like

https://github.com/rust-lang/rustfmt

Reviewers: mnauwelaerts

Subscribers: dhaumann, univerz, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D22963