Promotional pricing query methods

[AugustMiller]

Description

Adds PurchasableQuery::hasPromotionalPrice() to aid in discovering purchasables with a promotional price less than their regular price. This is designed as a replacement for the hasSales() query parameter for legacy Commerce applications.

{% set variantsWithPromotionalPrices = craft.variants()
  .hasPromotionalPrice(true) %}

{% set productsOnSale = craft.products()
  .hasVariant(variantsWithPromotionalPrices)
  .all() %}

Our Listing Products On Sale Knowledge Base article covers some DIY versions of this method—and a few advanced arithmetic queries! (I'll update this after this PR is merged.)

We've talked internally about what being “on promotion” actually means—currently, the comparisons are done against catalog pricing—but there are definitely developers and store managers who would expect that the system looks at the base prices (i.e. base price > price, or base price > promotional price), not just the fully-resolved catalog prices!

The existence of a base promotional price is a bit more challenging to detect (as far as query complexity goes), but I am pretty confident that the result would be the same… whether you check for an empty basePromotionalPrice or price = promotionalPrice would net the same results, due to the way purchasable prices are translated into the catalog pricing table (and then normalized/aggregated during the query).