Annotate return type of `createExpressServer` with more narrow type

[TokugawaTakeshi]

Description

Currently, createExpressServer function returns any.
It is the serious type safety crack for the application.

Proposed solution

I suppose, it must be Express type from express package or the superset of the Express.

[attilaorosz]

Indeed that’s something we could improve. Could be a good task for a first contributor.