Add GitHub action to run some automated checks

[robbieaverill]

Example of some checks that we can run:

• Linting (we'll need to add a linter and lint the project first)
• yarn deploy - this is what Netlify/Vercel run to deploy the project

We often have pull requests merged that break the one-click deployment pipelines, but we don't discover this until that happens and somebody reports it (#136). This could be avoided by adding continuous integration checks for the deployment command, which is essentially the same locally as it is in the hosting provider.

Linting is a nice to have - the deployment command is the higher value addition for now.