Change enhance function return type to void | Promise<void>

[Stadly]

closes #15700

The enhance function can be (and usually is) async. The return type must therefore not be void but void | Promise<void> to prevent triggering @typescript-eslint/no-misused-promises

The return type was changed from void | Promise<void> to void in #15530, which I think was a regression. This PR changes it back.

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