Allow the 'unused' category of lints (#2571)

These sort of warnings can be distracting when commenting out a few
lines of code or demonstrating some other concept. They can be
re-enabled for a code block with `warnunused`.

I filed https://github.com/rust-lang/mdBook/issues/2527 to get behavior
like this upstream.

src/types-and-values/variables.md

@@ -7,7 +7,7 @@ minutes: 5
 Rust provides type safety via static typing. Variable bindings are made with
 `let`:
 
-```rust,editable
+```rust,editable,warnunused
 fn main() {
     let x: i32 = 10;
     println!("x: {x}");
@@ -21,6 +21,10 @@ fn main() {
 - Uncomment the `x = 20` to demonstrate that variables are immutable by default.
   Add the `mut` keyword to allow changes.
 
+- Warnings are enabled for this slide, such as for unused variables or
+  unnecessary `mut`. These are omitted in most slides to avoid distracting
+  warnings. Try removing the mutation but leaving the `mut` keyword in place.
+
 - The `i32` here is the type of the variable. This must be known at compile
   time, but type inference (covered later) allows the programmer to omit it in
   many cases.