feature: better return types on instanceToPlain

[alvissraghnall]

Description

Excerpt from a previous issue [cos I share their thought process]:

class MyClass {
  @Expose({ name: 'desc', toPlainOnly: true })
  description: string
}

const x = { description: 'asc' }
const y = plainToInstance(MyClass, x)
const z = instanceToPlain(y) // type: Record<string, any>;

Proposed solution

I'd have to go through the codebase. Just documenting this first so I could fall back to it in a minute.

[muriloCoelho1212]

I'm having the same problem. The only way, apparently, is using as, but I don't know if that's the best way to do this. I think it would be more interesting if instanceToPlain returned the corresponding type