Displays the ProductVariant.stockLevel as either 'IN_STOCK', 'OUT_OF_STOCK' or 'LOW_STOCK'.
Low stock is defined as a saleable stock level less than or equal to the lowStockLevel as passed in
to the constructor (defaults to 2).
StockDisplayStrategy(ctx: RequestContext, productVariant: ProductVariant, saleableStockLevel: number) => string