Improve support for single entities

[mshima]

Fixes #13833

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[ecostanzi]

It works! But it prompts Do you want to update the entity? This will replace the existing files for this entity, all your custom code will be overwritten for each entity. It should prompt it just for the first one.

This is what I got by trying to generate the Tag entity with jhipster entity Tag --single-entity (the other two entities are Blog and Post, i'm using this jdl)

EDIT: same behaviour (multiple prompts) also without using --single-entity.

[pascalgrimaud]

We really need to document all these new flags and options