Enhance Object.assign static method signature

[falsandtru]

Fixes #12109

[dead-claudia]

There's a glitch: Object.assign mutates the target assignee directly. So it should really look something like this (which is actually correct mod a missing object | Function type constraint where object is implemented in this PR):

interface Object {
    assign<T>(target: T, ...sources: Partial<T>): T;
}

[mhegazy]

@sandersn is working on a fix for this as tracked by #11100. I would rather wait for that change.