feat(mise): support Rust idiomatic version files

[iloveitaly]

Motivation

Rust projects using rust-toolchain.toml should have their requested toolchain and rustup installation options honored through mise.

Description

• Enable Rust idiomatic version-file discovery in mise.
• Copy rust-toolchain.toml into the mise install step so channel, profile, components, and targets are applied.
• Document Rust toolchain parsing and the supported configuration file.

Test

• Built examples/rust-custom-toolchain and verified rust-toolchain.toml resolves Rust 1.59 through the idiomatic version-file path.
• Built examples/rust-custom-version and verified the Cargo.toml rust-version fallback remains unchanged.

Links

• https://mise.jdx.dev/configuration.html#idiomatic-version-files