expose CopyState to Vue template

Author: Jesse Haigh

src/components/ContentNode/CodeListing.vue

@@ -30,9 +30,10 @@
         :aria-label="$t('icons.copy')"
         :title="$t('icons.copy')"
       >
-        <CopyIcon v-if="copyState === 'idle'" class="copy-icon"/>
-        <CheckmarkIcon v-if="copyState === 'success'" class="checkmark-icon"/>
-        <CrossIcon v-if="copyState === 'failure'" class="cross-icon"/>
+        <CopyIcon v-if="copyState === CopyState.idle" class="copy-icon"/>
+        <CheckmarkIcon v-else-if="copyState === CopyState.success" class="checkmark-icon"/>
+        <CrossIcon v-else-if="copyState === CopyState.failure" class="cross-icon"/>
+
       </button>
       <!-- Do not add newlines in <pre>, as they'll appear in the rendered HTML. -->
       <pre><CodeBlock><template
@@ -83,6 +84,7 @@ export default {
     return {
       syntaxHighlightedLines: [],
       copyState: CopyState.idle,
+      CopyState,
     };
   },
   props: {