Change language to rust to let pre-commit bootstrap rust

[leoleoasd]

• Change language to rust to let pre-commit bootstrap rust
• Add an empty cargo.toml to fool the build system

Currently, this would fail on pre-commit.ci since it doesn't declare itself that it requires cargo.

See: before, after.

cargo fmt will work now. cargo clippy still won't work since it requires access to the internet.

[frank1010111]

this would be really nice

[eirnym]

I prefer this as an option as exact version of rust is important. E.g. some projects use nightly starting from specific version and cargo fmt would not recognize formatting options.