Streamdown 2.3 — Refreshed design and interactive playground

Streamdown 2.3 refreshes the markdown streaming renderer with a unified design and new developer tools.

• •Nested-card design applied to tables, code blocks, and Mermaid diagrams for visual consistency.
• •Action buttons stay sticky on scroll; code blocks render plain text immediately to reduce latency before syntax highlighting loads.
• •Interactive playground supports real-time testing with custom markdown and editable props.
• •useIsCodeFenceIncomplete hook detects in-progress fenced code blocks during streaming; tables support copying as Markdown.
• •Plugin docs for CJK, Math, and Mermaid consolidated into dedicated pages; full bug backlog cleared.