feature: Add Plain Typing.

[BenMcLean981]

Description

I think its a little odd that with Typescript all we know about a plain object is that its a of type Record<string, any>

For example:

import { classToPlain } from "class-transformer;

class Point {
    x: number;
    y: number;
    constructor(x: number, y: number) {
        this.x = x;
        this.y = y;
    }
}

plain = classToPlain(new Point(5, 4));
/* plain is of type Record<string, any>
  * But we in our heads sort of know that plain should be of type
  * {
  *     x: number;
  *     y: number;
  * }
  * /

Proposed solution

Before finding this library I was working on a generic type that would find properties to be carried over to a "plain" functionless object.

type PrimitiveKeys<T> = {
  [key in keyof T]: T[key] extends Function | undefined ? never : key;
}[keyof T];

type Plain<T> = {
  [key in PrimitiveKeys<T>]: T[key];
};

type PlainPoint = Plain<T>; //
{
    x: number;
    y: number;
}

The problems I ran into with this were with nested arrays and objects. I'm not super great with typescript typings so Im not really sure what the best way to fix this is (Which is why I'm offloading the functionality to class-transformer).

[ciriousjoker]

Perhaps you could add some sort of annotation to the class that defines what it converts into.
If that doesn't work, please add a generic type so we can do it manually.

---

Workaround:

const test = classToPlain(obj) as MyInterface;