[feat] add an Add to Cursor button to the README

[csells]

Cursor supports a shortcut for adding an MCP server via a link that triggers the desktop client. It would be great to have an "Add to Cursor" button in the README for this package that provided one-click dart_mcp_server configuration.

Here's their catalog of officially supported MCP servers as an example: https://docs.cursor.com/tools

[jakemac53]

Oh cool! We can definitely add that.