Add line to have app.wpjson work without Tailwind

[taylorgorman]

Per the @roots/sage Bud extension documentation page I've linked, .enable() is necessary when .useTailwind*() are not present. I had to google around to find out why my theme.json stopped generating.

To test this, I

1. Created a fresh roots/sage project
2. Changed a value in app.wpjson.setSettings() in bud.config.js, ran yarn build and saw the change in theme.json
3. Removed the three .useTailwind*() lines
4. Changed a value in app.wpjson.setSettings(), yarn build, no change in theme.json
5. Added .enable()
6. Changed a value in app.wpjson.setSettings(), yarn build, did see change in theme.json

I added the last sentence with the Bud docs link for proof and help, but feel free to remove if you'd rather keep the page shorter and more focused.

[retlehs]

Thank you!