Update liquid file names to match new Shopify theme names

[joshgrime]

Hi,

Shopify's updated theme files no longer follow the same naming conventions we have in this repository.

The liquid file names that have changed are listed below:

search.liquid --> main-search.liquid
product.liquid --> main-product.liquid
collection.liquid --> main-collection-product-grid.liquid

Furthermore, checkout.liquid is now deprecated. A Shopify pixel must be used to track the customer's checkout flow. We should provide a script for customers to use to continue tracking checkout events in Bloomreach.