Rust highlighting not working

[xeals]

The highlighter won't apply to Rust code.

Example:

```rust
fn main() {
    println!("WIP");
    let x = 34;
}
```

On Github, this renders as:

fn main() {
    println!("WIP");
    let x = 34;
}

On Grav, larger examples are auto-detected as either PHP or nginx in my experience. The class for the code block is parsed correctly and set to language-rust, but the highlighter doesn't apply (other languages use e.g., hljs php or hljs nginx).

HL version 1.4.0, Grav version 1.3.10, running in Docker.