feat: Add type information to `__assign(…)` helper function

[ExE-Boss]

This adds typed overloads to __assign, along with a TSDoc comment copied from lib.es2015.core.d.ts#ObjectConstructor.assign(…).

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review?(@DanielRosenwasser)

[DanielRosenwasser]

Is there any particular reason you're looking to type these more strictly?

[ExE-Boss]

Well, I’d like to eventually add type information to all TSLib helpers.

[DanielRosenwasser]

But is there a benefit? This code generally doesn't get consumed by users, and specifically this function has a hard time being accurately typed.

[ExE-Boss]

Well, this function acts as an alias for Object.assign if it exists, and uses an approximate implementation if it doesn’t, like in an ES3 environment, which is why I just copied the type information from that.

Arguably, it should be defined as:

export declare var __assign: typeof Object.assign;