Allow attaching browser tabs to chat to trigger sharing

[kycutler]

Similar to other editors, we should support an explicit attach action. This would trigger sharing and possibly send the page snapshot (and/or current selection) as context

[vs-code-engineering]

Hi @kycutler. As a member of the team, you can help us triage this issue by referring to https://github.com/microsoft/vscode-copilot/wiki/Copilot-Inbox-Tracker and assigning an owner directly.