Polyfill Promise.withResolvers().

This hopefully improves compatibility with old Safari versions and Hermes (React Native).

Unfortunately it didn't seem easy to extend the tests to cover React Native. There is a vitest-react-native package, but it looks little-used and unmaintained, so I didn't want to install it.

Might help with #91, though I won't declare that one "fixed" until we actually have tests proving it.

Author: kentonv

.changeset/six-banks-pull.md

@@ -0,0 +1,5 @@
+---
+"capnweb": patch
+---
+
+Polyfilled Promise.withResolvers() to improve compatibility with old Safari versions and Hermes (React Native).

src/core.ts

@@ -13,6 +13,20 @@ if (!Symbol.asyncDispose) {
   (Symbol as any).asyncDispose = Symbol.for('asyncDispose');
 }
 
+// Polyfill Promise.withResolvers() for old Safari versions (ugh), Hermes (React Native), and
+// maybe others.
+if (!Promise.withResolvers) {
+  Promise.withResolvers = function<T>(): PromiseWithResolvers<T> {
+    let resolve: (value: T | PromiseLike<T>) => void;
+    let reject: (reason?: any) => void;
+    const promise = new Promise<T>((res, rej) => {
+      resolve = res;
+      reject = rej;
+    });
+    return { promise, resolve: resolve!, reject: reject! };
+  };
+}
+
 let workersModule: any = (globalThis as any)[WORKERS_MODULE_SYMBOL];
 
 export interface RpcTarget {