Adding fill mode forwards to anim-fade-out (#1450)

* Adding fill mode forwards to anim-fade-out

* Create neat-poems-flow.md

Author: jonrohan

.changeset/neat-poems-flow.md

@@ -0,0 +1,5 @@
+---
+"@primer/css": patch
+---
+
+Adding fill mode forwards to anim-fade-out

src/utilities/animations.scss

@@ -25,6 +25,7 @@
 .anim-fade-out {
   animation-name: fade-out;
   animation-duration: 1s;
+  animation-fill-mode: forwards;
   animation-timing-function: ease-out;
 
   &.fast {