add `nim-toolchain.toml` toolchain override file support.
There is `rust-toolchain.toml` in rust ecosystem. When use arbitrary version of `cargo` to build a Rust project with `rust-toolchain.toml` It will automatically communicate with `rustup` and select the specific toolchain defined in `rust-toolchain.toml` to build the project. This is approach is very convenient and user-friendly for development.

For maximum compatibility. It would be awesome to introduce such kind of mechanism in `Nim` ecosystem. It would ensure that a Nim project is always successful to build. 