feat: add separator selector dropdown to Breadcrumb stories

[garrity-miepub]

• Add separatorMap with 6 separator options: chevron, slash, arrow, guillemet, pipe, dot
• Create separatorName dropdown control to replace ReactNode separator prop
• Hide original separator prop from controls table (was showing 'Set object' button)
• Add meta-level render function to convert separatorName to separator ReactNode
• Update Default, TwoItems, and WithSlashSeparator stories to use separatorName
• Import ChevronRight from lucide-react for chevron separator option
• Create BreadcrumbStoryArgs type extending BreadcrumbProps with separatorName

This fixes the 'Objects are not valid as a React child' error that occurred when users clicked 'Set object' on the separator control, which created an empty {} that React couldn't render.

breadcrumb.mov

[cloudflare-workers-and-pages]

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