Port TweetSearch demo to use ES modules

Replace the non-standard .pragma library style with the standard ES
modules and export statements for functions used from the outside.

Change-Id: Idd6bd0a5316edb690337b1b750c93060207d797b
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
Reviewed-by: Paul Wicking <paul.wicking@qt.io>