1.3.0

What's new

• Catalogue::extend() — a seam another addon can contribute priced things through.
  goldnead/statamic-offers uses it so that an upsell with its own price resolves like any other
  product, and every guard in here applies to it unchanged. The configured catalogue always wins:
  an addon may add, never reprice what the site has already decided.
• Checkout::start(..., $returnUrl) — where the provider sends the buyer back to. A funnel
  passes its own page, because a buyer who returns outside the flow they were walking has been
  dropped halfway through a purchase, and whatever was meant to follow the sale never happens.