build(docs-infra): add @default tag-def support (angular#40404)

This tag-def is being used in the `platform-server` source code,
and was causing warnings in the doc-gen. Adding support
prevents the warning from being shown.
Other than that this change has no effect on the generated
documentation, since the `@default` tag is not used in any
templates.

PR Close angular#40404

Author: petebacondarwin

aio/tools/transforms/angular-api-package/tag-defs/default.js

@@ -0,0 +1,3 @@
+module.exports = function() {
+  return {name: 'default'};
+};