Add pot file extraction (OSOE-927)

[sarahelsaig]

Add code that extracts the various [[ ... ]] expressions from Vue SFCs and generates GNU gettext PO template files (*.pot) that OC can understand. This could be a standalone program, but it would be better to do as a followup to OrchardCoreContrib/OrchardCoreContrib.PoExtractor#101, as a plugin to extractpo.

Jira issue