Support new parameters and globals for a11y

Closes #

What I did

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

[ ] stories
[ ] unit tests
[ ] integration tests
[ ] end-to-end tests

Manual testing

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

Documentation

[ ] Add or update documentation reflecting your changes
[ ] If you are deprecating/removing a feature, make sure to update MIGRATION.MD

Checklist for Maintainers

[ ] When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli-storybook/src/sandbox-templates.ts
[ ] Make sure this PR contains one of the labels below:

Available labels
- `bug`: Internal changes that fixes incorrect behavior. - `maintenance`: User-facing maintenance tasks. - `dependencies`: Upgrading (sometimes downgrading) dependencies. - `build`: Internal-facing build tooling & test updates. Will not show up in release changelog. - `cleanup`: Minor cleanup style change. Will not show up in release changelog. - `documentation`: Documentation **only** changes. Will not show up in release changelog. - `feature request`: Introducing a new feature. - `BREAKING CHANGE`: Changes that break compatibility in some way with current major version. - `other`: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

_core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>_

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Greptile Summary

Based on the provided files and changes, here's a concise summary of the pull request:

Enhanced the Storybook a11y addon with new configuration options and improved testing capabilities.

Added new warnings parameter to classify accessibility violations by impact level ('minor', 'moderate', 'serious', 'critical')
Moved manual flag from parameters to globals for better configuration control
Added Vitest integration for automated accessibility testing with proper error/warning handling
Updated documentation to reflect new configuration patterns and testing capabilities
Added comprehensive test coverage for new functionality in A11yContext and preview components

storybookjs / storybook