[Select] Simpler onChange event.target logic

[oliviertassinari]

Closes #12205. Using event.target.dataset doesn't make much sense in this context. The target could be coming from any element that triggered the click event. People can use event.currentTarget.dataset instead.

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event.target = { ...target, value, name };

The current spread syntax is pointless. The spread operator is using the iteration protocols to navigate over elements and collect the results (with arrays) or to copy enumerable properties (with objects).
But:

{...document.createElement('div')} // === {}

Instead, let's make a bold move. Yes, we try to make the event match the native change event at the cost of altering the event.

event.target = { value, name };

[rgallison]

People can use event.currentTarget.dataset instead.

Trying to do so with a TextField select and currentTarget is null. We ended up mutating the value of the options to access additional information about the option clicked