Add missing chevron-down.svg

Fixes #8988

wagtail/admin/static_src/wagtailadmin/images/icons/chevron-down.svg

@@ -0,0 +1,4 @@
+<svg id="icon-chevron-down" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16">
+    <!-- Font Awesome Free 5.15.3 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) -->
+    <path d="M6.035 7.195a.644.644 0 00.902 0l5.333-5.304c.246-.274.246-.684 0-.93l-.63-.629c-.245-.246-.656-.246-.93 0L6.5 4.543 2.262.332c-.274-.246-.684-.246-.93 0L.703.961c-.246.246-.246.656 0 .93l5.332 5.304z"></path>
+</svg>