Doesn't work with stable toolchain out of the box

[tomkv]

Steps to reproduce:

1. Install current stable rustc, rust-std, rust-src, rust-docs, rust-analysis, rls-preview, cargo (currently 1.23.0), using rustup component add rls-preview rust-analysis rust-src. Put .cargo/bin into the $PATH, make sure the changed path is applied to the current session.
2. Install vscode and rls-vscode from the marketplace (currently 0.3.2). No user preferences are set.
3. Create a new, empty project with cargo init --bin
4. Open the project folder in vscode and open the generated src/main.rs in vscode.

These steps reproduce what a new user would do.

Result:

The user will get the following messages:

• Warn: RLS could not set RUST_SRC_PATH for Racer because it could not read the Rust sysroot. [Close]
• Info: nightly toolchain not installed. Install? [Yes][Close] ([Close], we want to avoid breakage when the nightly is missing rls)
• Error: Couldn't start client Rust Language Server. [Close]

Expected result:

When the stable release with all required components is available, the addon should a) not to offer to install nightly toolchain b) use the toolchain available.

Possible workaround:

The user can set the preference rust-client.channel to stable. However, the initial value is not autodetected, and the default is nightly, leading to the confusion.

[Xanewok]

Autodetection was implemented in #220, so this should be more or less fixed when the code including the change is published.

[tomkv]

Great, thanks!