Unroll arrow styling into SASS

There is so much modification of the `menu-arrow` mixin that it's
not worthwhile abstracting it out (and we're moving away from mixins).

src/styles/annotator/components/AdderToolbar.scss

-@use '../mixins/molecules';
 @use '../variables' as var;
 
 $adder-transition-duration: 80ms;
@@ -44,7 +43,11 @@ $adder-transition-duration: 80ms;
 }
 
 .AdderToolbar__arrow {
-  @include molecules.menu-arrow;
+  position: absolute;
+  z-index: 2;
+  color: var.$color-border;
+  fill: var.$color-background;
+
   // Horizontal centering, part 1
   left: 50%;
 
@@ -54,12 +57,8 @@ $adder-transition-duration: 80ms;
 
   &--down {
     // These transforms:
-    // 1) Reimplement the "down" variant of the menu-arrow mixin,
-    //    rotating the arrow pointer SVG.
-    //    Having this here is necessary because all transforms for an element
-    //    must be in a single rule, and we also need to:
-    // 2) Translate -50% (left) on the X axis to complete horizontal centering
-    // 3) Translate up (Y) by 1px to eliminate a faint border line visible in Safari
+    // 1) Translate -50% (left) on the X axis to complete horizontal centering
+    // 2) Translate up (Y) by 1px to eliminate a faint border line visible in Safari
     transform: rotateX(180deg) translateX(-50%) translateY(1px);
   }