Enable doctag in syntax highlighter

Example:

/**
 * @param test
 */

Change-Id: I9aeafbb8c29428f18e29a80adafab4e228477d4b

polygerrit-ui/app/elements/diff/gr-syntax-layer/gr-syntax-layer.js

@@ -99,6 +99,7 @@
     'gr-diff gr-syntax gr-syntax-attribute': true,
     'gr-diff gr-syntax gr-syntax-built_in': true,
     'gr-diff gr-syntax gr-syntax-comment': true,
+    'gr-diff gr-syntax gr-syntax-doctag': true,
     'gr-diff gr-syntax gr-syntax-function': true,
     'gr-diff gr-syntax gr-syntax-keyword': true,
     'gr-diff gr-syntax gr-syntax-link': true,

polygerrit-ui/app/elements/diff/gr-syntax-themes/gr-syntax-theme.html

@@ -103,6 +103,9 @@ limitations under the License.
       .gr-syntax-param {
         color: var(--syntax-param-color);
       }
+      .gr-syntax-doctag {
+        font-weight: var(--syntax-doctag-weight);
+      }
     </style>
   </template>
 </dom-module>

polygerrit-ui/app/styles/themes/app-theme.html

@@ -143,6 +143,7 @@ limitations under the License.
   --syntax-selector-attr-color: #FA8602;
   --syntax-template-tag-color: #FA8602;
   --syntax-param-color: var(--primary-text-color);
+  --syntax-doctag-weight: bold;
 }
 @media screen and (max-width: 50em) {
   :root {

polygerrit-ui/app/styles/themes/dark-theme.html

@@ -103,6 +103,7 @@ limitations under the License.
       --syntax-regexp-color: #F77669;
       --syntax-selector-attr-color: #80CBBF;
       --syntax-template-tag-color: #C792EA;
+      --syntax-doctag-weight: bold;
 
       background-color: var(--view-background-color);
     }