Add ExecutionContext as global _value_ to test environment types

Miniflare injects the `ExecutionContext` class into its unit testing
environments, so they can be constructed and passed to imported
module event handlers. Previously, this type was only available as
an `interface` from `@cloudflare/workers-types`, causing a type error
whenever it was constructed. With this change, it's now declared as
a value when including `{jest,vitest}-environment-miniflare/globals`
in the `tsconfig.json` `types` array.

Author: mrbbot

packages/shared-test-environment/globals.d.ts

@@ -78,6 +78,12 @@ declare global {
   function getMiniflareDurableObjectIds(
     namespace: string
   ): Promise<DurableObjectId[]>;
+
+  // eslint-disable-next-line no-var
+  var ExecutionContext: {
+    prototype: ExecutionContext;
+    new (): ExecutionContext;
+  };
 }
 
 // Taken from `undici` (https://github.com/nodejs/undici/tree/main/types) with