The purpose of this button is to copy a message source, including the markdown. The solution uses a recommended approach https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Interact_with_the_clipboard of interacting with the clipboard.
Linear task: https://linear.app/comm/issue/ENG-2332/copy-message-button-on-web
In this diff we're not giving any feedback to the user, especially we don't display a different tooltip when a message is successfully copied. https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1170%3A78271&t=LDQhPB76aimwoBhD-0 This will be handled later in the stack.