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/layout/flow/inline/construct.rs

@@ -71,7 +71,7 @@ pub(crate) struct InlineFormattingContextBuilder {
     /// The traversal is at all times as deep in the tree as this stack is,
     /// which is why the code doesn't need to keep track of the actual
     /// container root (see `handle_inline_level_element`).
-    //_
+    // _
     /// When an inline box ends, it's removed from this stack.
     inline_box_stack: Vec<InlineBoxIdentifier>,