Helpers: Add missing type definitions.

[Heaust-ops]

Description

Directional Light Helper, Spot Light Helper and Hemisphere Light Helper didn't have types,

So they inherited their parent's type "Object3D",

which isn't consistent with all the other helpers like Point Light Helper which has a type of "PointLightHelper".

This PR gives them their own types.

[Mugen87]

The type property is actually only needed for serialization/deserialization which is not supported for helpers in general.

However, it's still better for consistency reasons to add the remaining types. So I think the PR is fine.

[Heaust-ops]

That's awesome ty :D
I was actually using types to detect if raycast intersects were helpers or not and if they were, what kind.