Add automated a11y testing

[echappen]

Now that we're getting into building UI components, we should incorporate accessibility (a11y) testing into the repo.

pa11y-ci is the preferred method in the Engineering Practices guide, but I'm open to trying Chrome's Lighthouse CI.

Every page/component tested should meet or exceed WCAG 2.1 AA standards.

Acceptance criteria

• [ ] a check is added to GH actions for every pull request
• [ ] this check runs an a11y scanner tool on our components
• [ ] failure of this check blocks merging the PR

[echappen]

Ask Pages on their current work

[jduss4]

@sknep sent their latest: https://github.com/cloud-gov/pages-cf-build-tasks/tree/main/tasks/a11y

[echappen]

This will have the same limitations as with E2E testing (#347) because mock CF resources will need to be stood up and torn down during the CI runs.