Dynamically import the fetch ponyfill in contexts where it isn't available

We're currently always statically importing `cross-fetch` to use as a fetch polyfill. The only environment we actually need to polyfill fetch in is node 16 which is used by old gadget apps on fv1 -- we already only support browsers that have `fetch` defined globally, and node 18 in fv2 has it as well. So, most folks are paying a bundle size penalty for no great reason.

Instead, let's move this chunk out into a separate import that is only dynamically imported if needed. This will happen only on node16 which will be nice and snappy, and shouldn't cause a request waterfall in browsers. You'll notice that vite picks up on this dynamic import and creates a new output chunk for it which is what we want for the browser as well.

Before:

```
vite v4.4.7 building for production...
✓ 173 modules transformed.
dist/api-read-stats.html      210.79 kB │ gzip: 46.88 kB
dist/test-bundle-api-read.js  211.80 kB │ gzip: 37.36 kB
dist/api-read-stats.html       211.39 kB │ gzip: 46.93 kB
dist/test-bundle-api-read.cjs  138.54 kB │ gzip: 30.51 kB
✓ built in 769ms
vite v4.4.7 building for production...
✓ 449 modules transformed.
dist/react-read-stats.html      302.29 kB │ gzip: 56.83 kB
dist/test-bundle-react-read.js  316.93 kB │ gzip: 66.53 kB
dist/react-read-stats.html       306.22 kB │ gzip: 56.92 kB
dist/test-bundle-react-read.cjs  213.27 kB │ gzip: 55.67 kB
✓ built in 1.13s
vite v4.4.7 building for production...
✓ 983 modules transformed.
dist/shopify-read-stats.html      515.87 kB │ gzip:  77.42 kB
dist/test-bundle-shopify-read.js  636.23 kB │ gzip: 125.20 kB
dist/shopify-read-stats.html       516.37 kB │ gzip:  77.42 kB
dist/test-bundle-shopify-read.cjs  447.28 kB │ gzip: 106.73 kB
✓ built in 2.29s
```

After:

```
vite v4.4.7 building for production...
✓ 173 modules transformed.
dist/api-read-stats.html           210.92 kB │ gzip: 46.91 kB
dist/browser-ponyfill-a950d05c.js   12.53 kB │ gzip:  3.50 kB
dist/test-bundle-api-read.js       199.78 kB │ gzip: 34.45 kB
dist/api-read-stats.html            211.51 kB │ gzip: 46.98 kB
dist/browser-ponyfill-0b1c63f2.cjs    8.88 kB │ gzip:  3.07 kB
dist/test-bundle-api-read.cjs       130.06 kB │ gzip: 28.07 kB
✓ built in 774ms
vite v4.4.7 building for production...
✓ 449 modules transformed.
dist/react-read-stats.html         302.31 kB │ gzip: 56.84 kB
dist/test-bundle-react-read.js       0.09 kB │ gzip:  0.10 kB
dist/browser-ponyfill-aae43e35.js   12.44 kB │ gzip:  3.50 kB
dist/react-read-b8f94e10.js        305.18 kB │ gzip: 63.69 kB
dist/react-read-stats.html          306.31 kB │ gzip: 56.96 kB
dist/test-bundle-react-read.cjs       0.17 kB │ gzip:  0.16 kB
dist/browser-ponyfill-1992ee4c.cjs    8.83 kB │ gzip:  3.08 kB
dist/react-read-e3b5cb04.cjs        204.97 kB │ gzip: 53.20 kB
✓ built in 1.16s
vite v4.4.7 building for production...
✓ 983 modules transformed.
dist/shopify-read-stats.html       514.75 kB │ gzip:  77.59 kB
dist/test-bundle-shopify-read.js     0.12 kB │ gzip:   0.12 kB
dist/browser-ponyfill-d3f0b39a.js   12.45 kB │ gzip:   3.50 kB
dist/shopify-read-62230a97.js      624.51 kB │ gzip: 122.38 kB
dist/shopify-read-stats.html        515.26 kB │ gzip:  77.60 kB
dist/test-bundle-shopify-read.cjs     0.19 kB │ gzip:   0.17 kB
dist/browser-ponyfill-8417ec9c.cjs    8.83 kB │ gzip:   3.08 kB
dist/shopify-read-25c98907.cjs      439.00 kB │ gzip: 104.22 kB
```

Another 6% bundle size reduction

Author: airhorns

packages/api-client-core/src/GadgetConnection.ts

@@ -1,6 +1,6 @@
 import type { ClientOptions, RequestPolicy } from "@urql/core";
 import { Client, cacheExchange, fetchExchange, subscriptionExchange } from "@urql/core";
-import fetchPolyfill from "cross-fetch";
+
 import type { ExecutionResult } from "graphql";
 import type { Sink, Client as SubscriptionClient, ClientOptions as SubscriptionClientOptions } from "graphql-ws";
 import { CloseCode, createClient as createSubscriptionClient } from "graphql-ws";
@@ -39,7 +39,7 @@ export interface GadgetConnectionOptions {
   websocketsEndpoint?: string;
   subscriptionClientOptions?: GadgetSubscriptionClientOptions;
   websocketImplementation?: any;
-  fetchImplementation?: typeof fetchPolyfill;
+  fetchImplementation?: typeof globalThis.fetch;
   environment?: "Development" | "Production";
   requestPolicy?: ClientOptions["requestPolicy"];
   applicationId?: string;
@@ -71,7 +71,7 @@ export class GadgetConnection {
   private subscriptionClientOptions?: SubscriptionClientOptions;
   private websocketsEndpoint: string;
   private websocketImplementation: any;
-  private _fetchImplementation: typeof fetchPolyfill;
+  private _fetchImplementation: typeof globalThis.fetch;
   private environment: "Development" | "Production";
   private exchanges: Required<Exchanges>;
 
@@ -95,7 +95,10 @@ export class GadgetConnection {
     } else if (typeof window != "undefined" && window.fetch) {
       this._fetchImplementation = window.fetch.bind(window);
     } else {
-      this._fetchImplementation = fetchPolyfill;
+      this._fetchImplementation = async (...args: [any]) => {
+        const { fetch } = await import("cross-fetch");
+        return await fetch(...args);
+      };
     }
     this.websocketImplementation = options.websocketImplementation ?? globalThis?.WebSocket ?? WebSocket;
     this.websocketsEndpoint = options.websocketsEndpoint ?? options.endpoint + "/batch";
@@ -124,7 +127,7 @@ export class GadgetConnection {
     return this.currentTransaction?.client || this.baseClient;
   }
 
-  set fetchImplementation(implementation: typeof fetchPolyfill) {
+  set fetchImplementation(implementation: typeof globalThis.fetch) {
     this._fetchImplementation = implementation;
     this.resetClients();
   }