add `nim-toolchain.toml` toolchain override file support.

[evan0greenup]

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.

[xTrayambak]

This seems like a job for choosenim, than nimble. Nimble cannot manage your toolchain.