Add command to yank diagnostic under cursor

[rcorre]

#3786, adds a yank shortcut to the picker, but if my cursor is already over the diagnostic in question, I'd like a shortcut to just yank that without finding it in the picker.

I suppose the cursor could be over multiple diagnostics, but this is rare. In that case I think any of the following would be fine:

1. Yank all diagnostics joined
2. Open a picker
3. Open a dropdown