Make it easier to copy text from snack

When certain types of snack are issued, particularly errors, it can be useful to copy the text of the message to the clipboard. However, those snacks disappear on mouse-up, before one has a chance to do a Ctrl-C.

It would be great if it were possible to copy these messages to the clipboard.

(Alternatively, and this will eventually be a separate feature request, enable the user to view a log of snack messages.)

Start by reproducing a snack disappearing on mouse-up, then locate the TypeScript implementation responsible for displaying and dismissing snack messages. The change is done when the message text can be copied to the clipboard before dismissal, including for error snacks, without breaking normal snack behavior.

Written by the indexing model from the issue text.