[Fix] Fixes tags for assets

[mvaligursky]

The Asset class had both id and _id members, and in #7517 the _id had been removed.

Unfortunately the tags class still used it, and tagging stoped working. This makes tags use id.

[Maksims]

The hot-fix in the meantime:

Object.defineProperty(pc.Asset.prototype, '_id', {
    get: function () {
        return this.id;
    },
    set: function (value) {
        this.id = value;
    }
});