As we don't have selection inside the chat right now (which might be
complicated to implement, e.g. cross element selection and markdown
blocks), I think its viable to support copying the whole text of a
message using the message menu:

![image](https://github.com/zed-industries/zed/assets/53836821/6092dfed-0297-457e-9455-ba1e6190e288)

Release Notes:

- Added option to copy the text of a message within the chat