Added build:ts yarn script to typescript generator (#201)

no issue

- When creating a new typescript module in the Ghost monorepo, you have to manually add the `build:ts` script to the `package.json` to get things to work properly
- This commit just adds that command to the typescript generator so creating a new typescript module should work out of the box now.

packages/generator-slimer/generators/node/templates/package.json

@@ -8,7 +8,8 @@
   "types": "build/index.d.ts",<% } %>
   "scripts": {<% if (isTypescript) { %>
     "dev": "tsc --watch --preserveWatchOutput --sourceMap",
-    "build": "tsc",
+    "build": "yarn build:ts",
+    "build:ts": "tsc",
     "prepare": "tsc"<% } else { %>
     "dev": "echo \"Implement me!\""<% } %>
   },