Sharing feature could use a shared-text preview

### Your use case

#### What would you like to do?
Provide a preview (and optional caption) of the message I am sharing with others (especially in other rooms/spaces where context will rarely be apparent)

#### Why would you like to do it?
Because sharing is the lifeblood of collaboration, and the more explicit a Share action can be, the better. I'd even like the Sharing dialog to permit me to drag "Related links"" to it (but that can come later).

#### How would you like to achieve it?
1) Simply add a "Preview text" input that would include the first 180 chars of the item (and any already selected text). The user can edit to suit their context if needed.

2) add an optional "Caption" field for random call-outs (very important in many contexts)

To illustrate:


### Have you considered any alternatives?

I don't think there is a better approach. Sharing is the intent. so the more facilitated it can be, the better.

### Additional context

Quibble. I think the QR code and sharing icons on existing Sharing feature are far too large, Visually, they become the most important thing on the form, and they'd be to miss at a scale half the present size.