Update slug when product handle updates

[daun]

Updating a product handle in Shopify and resyncing into Statamic currently leads to disconnected variants — the products keep their old slug while the variants are updated to reference the new one. This proposed solution updates the entry slug to always match the Shopify handle. Requires the slug field in the control panel to be set to read-only or hidden. That's how we handle it at least.

Alternatively, one could allow updates to entry slugs inside Statamic, which would require a different change instead. This would/could tie into the overwrite setting, which would make overwriting the slug optional/configurable.

// Import Variant
- $this->importVariants($this->data['variants'], $this->data['handle']);
+ $this->importVariants($this->data['variants'], $entry->slug());