Allow target and rel attributes on links in the rich text editor

[IsraelOrtuno]

The link dialog in the rich text editor (Cmd/Ctrl + K) only accepts a URL. There's no way for an editor to mark a specific link as opening in a new tab, or to add rel="nofollow" / rel="sponsored" on a per-link basis.

We can code static workarounds like rules for external links but what's actually needed is a per-link decision made by the person writing the content.

[transportrefer]

Confirmed on EmDash 0.35 with Cloudflare: the editor DOM can show rel="noopener noreferrer nofollow", while the stored Portable Text link contains only href and the public renderer emits no rel. Please also consider ugc, and ensure these attributes survive editor round-trips and CLI/MCP updates.