tidy: Add a rule ensuring that // comments are followed by a space in Rust (#38698)

This shows up sometimes in code reviews, so it makes sense that tidy
enforces it. `rustfmt` supports this via comment normalization, but it
does many other things and is still an unstable feature (with bugs).

Testing: There are new tidy tests for this change.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/webgpu/gpuadapter.rs

@@ -178,7 +178,7 @@ impl GPUAdapterMethods<crate::DomTypeHolder> for GPUAdapter {
 
     /// <https://gpuweb.github.io/gpuweb/#dom-gpuadapter-isfallbackadapter>
     fn IsFallbackAdapter(&self) -> bool {
-        //TODO
+        // TODO
         false
     }