Add Vercel workflow

[ethitter]

Switches Vercel deploys to a GitHub action

Make sure you complete these items:

• [ ] Updated root CHANGELOG.md with summary of changes under Unpublished section
• [ ] npm run prod in this repo outputs expected changes (excepting the issue with re-ordered partials in larva-css algorithms partials - see LRVA-1885)
• [ ] If adding a new pattern, in the PR comment, included a screenshot and link to the static Vercel deployment
• [ ] If changes to build scripts or the Node.js server, tested changes in pmc-spark via a pre-release
• • If changes to build tools: npm scripts prod, lint, and dev scripts run as expected
• • If changes to Larva server: Static site generates as expected in a theme (avail. on a URL {brand}.stg.larva.pmcdev.io)