Refactor Store to expose `.prototype`

[rougeayrn]

Brief Description

Expose store.prototype to allow for easy addition of player-defined properties to store game objects.

Sample code

Reference Traxus' example workaround here, though obviously should not be used as the solution.

Does work in Javascript:

Object.defineProperty(Store.prototype, "contents", {
    get() {
        Object.entries(this);
    },
    enumerable: false,
    configurable: true
});

[DocDvorak]

Implemented by #294