Accessibility

## Epic

All Dojo 2 applications to be as accessible as possible. Giving developers the right tooling to identify issues with accessibility in their applications plus providing sound accessibility patterns and libraries.

Potential areas of development:

### Tooling
* accessibility linting tools for cli-build-app
* accessibility testing tools for cli-test-intern

### Patterns
* blockingElements
* inert
* focus management (for both widgets and elements)
* ~focus-ring~ focus-visible
* accessibility around menu's?
* review of existing widget patterns for a11y compliance
* review of dojo theme styles for a11y, with attention to the new WCAG rules