Emit new `merge` helper for fixed object spread semantics

[Andarist]

Since #32514 spreading always handle 2 objects at the time.

It has been suggested that because of that assign helper could be simplified to handle 2 arguments - instead of variadic amount - #32514 (comment) . I believe this would be slightly beneficial for the compiled output etc so I would like to propose pursuing this simplification and I can offer my intent to implement this. Would only need to know if there is something else that needs to be done for this except changing the code. I know tslib has to be updated, but ain't sure how its code is generated. Or are helpers just inlined there manually?

Requested description

I propose to add merge helper which would be implemented like this

export var __merge = function __merge(target, source) {
    for (var prop in source) if (Object.prototype.hasOwnProperty.call(source, prop)) target[prop] = source[prop];
    return target;
}

It's just a simplification over assign (which supports variadic list of arguments) - but this one handles only 2 arguments: target & source (and this is sufficient since #32514 )

[RyanCavanaugh]

@Andarist would you mind updating this with a more detailed inline description of what you're suggesting?

[Andarist]

done

[RyanCavanaugh]

Doesn't seem like this is a needed improvement given the current state of runtimes