Stability tracking - RLS

[nrc]

To ride the trains:

• [x] tests are testing on Rust CI (https://github.com/rust-lang/rust/pull/42146)
• [x] rename (possibly automatically) rls component to rls-preview
• [x] De facto unstable features are behind the unstable features option
• [x] Unstable features option is linked to nightly toolchain
• [x] Decide on name of the Rustup component (proposed: rls-preview)
• [x] Proper channel info from --version.
• [x] Use the rls-rustc shim (https://github.com/rust-lang-nursery/rls/issues/435)

1.0:

• [ ] Quality - waiting on bug reports from stable channel release
• [ ] Cargo project coverage - lib + bin, workspaces
• [ ] Better testing (better protocol coverage, prevent regressions due to compiler data)
• [ ] new features - TBD

[nrc]

cc https://github.com/rust-lang/rust/pull/44141