Refactor Toast as BaseComponent class with native CSS autohide

[anvme]

Description

Refactor Toast from a side-effect-only module (raw document.addEventListener) into a proper BaseComponent class — matching the pattern used by Alert, Button, Tab, and every other Bootstrap component

Motivation & Context

After this discussion, I shared a proposal for replacing JS-heavy components with native browser APIs, similar to what was already done with Modal in v6

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• [ x] Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• [ x] I have read the contributing guidelines
• [ x] My code follows the code style of the project (using npm run lint)
• [ x] My change introduces changes to the documentation
• [ x] I have updated the documentation accordingly
• [ x] I have added tests to cover my changes
• [ x] All new and existing tests passed

Live previews

• https://deploy-preview-42101--twbs-bootstrap.netlify.app/docs/6.0/components/toasts/

Related issues

https://github.com/orgs/twbs/discussions/42090