Add auto to POS button variant type

[henryStelle]

Background

Fixes https://github.com/shop/issues-retail/issues/33323.

The POS s-button documentation identifies auto as the default variant, but the public button variant type only allowed primary and secondary.

Solution

Add auto to the POS button variant type and generated documentation, and document it alongside the other supported variants.

Validation

• yarn build
• yarn type-check
• yarn lint
• Targeted TypeScript compile check for variant: 'auto'

The full yarn test -- packages/ui-extensions/ command is currently blocked by the missing react-test-renderer dependency in unrelated Checkout suites.

🎩

• Confirm a POS extension can type-check <s-button variant="auto">.

Checklist

• I have 🎩'd these changes
• I have updated relevant documentation

Forward-port of #4627, #4629, #4628, #4626

[fatbattk]

code lgtm but does auto actually work?

determines the button variant based on its context