CHECKOUT-9450: Lazy load payment strategies through essential build

[davidchin]

What/Why?

checkout-sdk-js currently bundles all payment, shipping, and checkout button strategies for every payment provider, regardless of whether they are enabled by the merchant. This leads to unnecessary code in the main bundle, increasing its size and reducing performance.

To address this problem, the following changes are introduced:

• Added a new export, @bigcommerce/checkout-sdk/essential, which excludes all payment, shipping, and checkout button strategies from the bundle.
• Added an ESM build, allowing the library to be fully tree-shakable.
• Updated strategy loading so that strategies are imported from @bigcommerce/checkout-sdk/integrations/* on the application side and passed into CheckoutService only when needed. This enables splitting strategies into separate chunks based on the component that loads them.

// App.tsx
import { createCheckoutService } from '@bigcommerce/checkout-sdk/essential';

const service = createCheckoutService();

// PayPalCommercePaymentMethod.tsx
import { createPayPalCommercePaymentStrategy } from '@bigcommerce/checkout-sdk/integrations/paypal-commerce';

await service.initializePayment({
    methodId: 'paypal',
    integrations: [createPayPalCommercePaymentStrategy],
});

• To reduce risk during the initial rollout, strategies will still be bundled as usual while the CHECKOUT-9450.lazy_load_payment_strategies experiment is active. We will compare the bundled strategies with those passed in from checkout-js and log any discrepancies in Sentry. Once we confirm that all passed-in strategies consistently match the existing ones, we will remove the bundled strategies.

Rollout/Rollback

Disable CHECKOUT-9450.lazy_load_payment_strategies experiment

Testing / Proof

The screenshots below show the improvements after the experiment is fully rolled out and the old code removed.

Before

716kb gzipped size for the initial scripts

After

546kb gzipped size for the initial scripts - ~24% improvement

This screenshot shows that payment methods still load as expected.