[kbn-plugin-generator] gitignore app.css if scss used (#25609) (#25612)

When generating a plugin with the plugin generator and scss is in use, we should add the generated `app.css` file to the .gitignore file.

Author: Spencer

packages/kbn-plugin-generator/sao_template/template/gitignore

@@ -1,3 +1,6 @@
 npm-debug.log*
 node_modules
 /build/
+<%_ if (generateScss) { -%>
+/public/app.css
+<%_ } -%>