feat: @set and related model fields

[davenewza]

@set and related model fields

@set has been limited to just setting a model field from actions inputs and ctx. This extends @set functionality to support looking up fields in related models in both create and update actions.

A nice example of this would be to set the price of a purchase order once-off from a product's standard price:

model Order {
    fields {
        price Decimal
        quantity Number
        product Product
        total Decimal @computed(order.price * order.quantity)
    }
    actions {
        create createOrder() with (product.id, quantity, customer.id) {
            @set(order.price = order.product.standardPrice)
        }
        update resetOrderPrice(id) {
            @set(order.price = order.product.standardPrice)
        }
    }
}

model Product {
    fields {     
        standardPrice Decimal
    }
}

Note that we do not support looking up 1:M values yet.