Add to .rules: Avoid creating mod.rs paths (#29174)

Release Notes:

- N/A

.rules

@@ -5,6 +5,7 @@
 * Prefer implementing functionality in existing files unless it is a new logical component. Avoid creating many small files.
 * Avoid using functions that panic like `unwrap()`, instead use mechanisms like `?` to propagate errors.
 * Be careful with operations like indexing which may panic if the indexes are out of bounds.
+* Never create files with `mod.rs` paths - prefer `src/some_module.rs` instead of `src/some_module/mod.rs`.
 
 # GPUI