Build externs file from info.json

This change adds all information that is needed to generate the externs
file to build/info.json, so tasks/generate-externs.js no longer needs
to spawn JSDoc.

buildcfg/jsdoc/info/api-plugin.js

@@ -0,0 +1,15 @@
+
+
+/**
+ * Handle the api annotation.
+ * @param {Object} dictionary The tag dictionary.
+ */
+exports.defineTags = function(dictionary) {
+
+  dictionary.defineTag('api', {
+    onTagged: function(doclet, tag) {
+      doclet.api = tag.text || 'experimental';
+    }
+  });
+
+};