Highlight focused diff file (#22870)

When a diff file has been focused through the URL ID fragment, it will
be highlighted.

![light
theme](https://user-images.githubusercontent.com/51889757/218283880-ce551cb3-0c21-4044-adb0-d9ceb8a15ec5.png)
![dark
theme](https://user-images.githubusercontent.com/51889757/218283885-e96e989a-4670-49b6-9733-fbd6fffabf20.png)

---------

Co-authored-by: Lunny Xiao <[email protected]>

Author: delvh

web_src/less/_review.less

@@ -317,3 +317,11 @@ a.blob-excerpt:hover {
   width: 72px;
   height: 10px;
 }
+
+.diff-file-box {
+  border-radius: .285rem; // Just like ui.top.attached.header
+}
+
+.diff-file-box:target {
+  box-shadow: 0 0 0 3px var(--color-accent);
+}