feat: computed fields and many to one relationships

[davenewza]

@computed and M:1 relationships

This PR provides support for referencing fields in a M:1 relationship in a @computed expressions. For example:

model Item {
    fields {
        quantity Number
        product Product
        total Decimal @computed(item.quantity * item.product.price)
    }
}

model Product {
    fields {
        name Text
        price Decimal
    }
}