Add "export" to ProcessMode enum

Coming in the 1.7.0 (not yet released) version of Arkindex.

assigned to @mlbonhomme

mentioned in issue arkindex/workers/export#9 (closed)

mentioned in merge request !604 (merged)

changed milestone to %0.4.0

added P2 label

changed the description

closed with merge request !604 (merged)

mentioned in issue #381 (closed)