Modify structured suggestion output

* On suggestions that include deletions, use a diff inspired output format
* When suggesting addition, use `+` as underline
* Color highlight modified span

compiler/rustc_errors/src/snippet.rs

@@ -177,4 +177,6 @@ pub enum Style {
     NoStyle,
     Level(Level),
     Highlight,
+    Addition,
+    Removal,
 }