Move placeholder styles to highlights bundle

This class is used by elements created by placeholder.js in the host
page, not a shadow root.

src/styles/annotator/annotator.scss

@@ -75,13 +75,6 @@
   }
 }
 
-.annotator-placeholder {
-  opacity: 0;
-  position: absolute;
-  top: 50%;
-  z-index: -1;
-}
-
 // this disables the width transition for the sidebar when
 // it is manually resized by dragging
 .annotator-no-transition {

src/styles/annotator/highlights.scss

@@ -78,3 +78,12 @@
     }
   }
 }
+
+// Placeholder element to which annotations for off-screen content in PDFs
+// is anchored.
+.annotator-placeholder {
+  opacity: 0;
+  position: absolute;
+  top: 50%;
+  z-index: -1;
+}