Feature Request: Add support for sharing files in a channel

[stufflebeam]

Similarly to pasting in #133 -- it would be helpful to have a + button to add files/snippets/etc. and also to be able to drag and drop files to share into vscode-chat like can be done in the native Slack app.

Adding a + button for snippets may be a first step towards #81 (which would be a fantastic addition itself).

[arjunattam]

thanks @stufflebeam - I agree adding the + button will be a good place to start. Happy to accept PRs for this

for drag/drop support, we are dependent on this issue upstream: microsoft/vscode#8794