Add #53664, public&export error to news (#56911)

Suggested by @simeonschaub
[here](#53664 (comment))

Author: LilithHafner

NEWS.md

@@ -46,6 +46,9 @@ Language changes
    behavior. Infinite loops that actually do things (e.g. have side effects
    or sleep) were never and are still not undefined behavior. ([#52999])
 
+ - It is now an error to mark a symbol as both `public` and `export`ed.
+   ([#53664])
+
 Compiler/Runtime improvements
 -----------------------------