Add MCP Apps into mcp-builder skill

[atlekbai]

MCP Apps are relatively new and the spec is mostly driven by Anthropic engineers so it would be great if mcp-builder skill would soon support new MCP App spec

[edkranz]

there is already one in the modelcontextprotocol marketplace, see
https://modelcontextprotocol.io/docs/extensions/apps#using-an-ai-coding-agent

[atlekbai]

@edkranz thanks! Ill try it out