Add service variant compatiablity

[thekp]

Resolves #2667

Overall change: Updated our example data for the storybook-helper package by adding variant prop into the storyProps object.

Code changes:

• updated the text-variants file to include a variant and service value for variant service.
• added logic to the withServiceKnob to pass a variant when it exists
• made the code more readable by adding constants at the top of the file
• install ramda to root package.json and use path to safely check for values in object

---

• I have assigned myself to this PR and the corresponding issues
• Automated jest tests added (for new features) or updated (for existing features)
• This PR requires manual testing

[sareh]

👍 Looks good. It adds variant support for Serbian/UKChina/Zhongwen.
Though I also worked on part of it with @thekp, so it'd be good to get other eyes on it too.

[FK78]

Branch must update CHANGELOG.md in psammead-storybook-helpers
Branch must update package-lock.json in psammead-storybook-helpers
Branch must update package.json in psammead-storybook-helpers