Use Fetch ponyfill in Node 18 to avoid issues with new global fetch

It's likely that some of these are just differences, rather than real
problems, but there's little downside in sticking with the ponyfill for
now (since we need it for older Node anyway) and it should let us get
everything working properly to fully support Node v18 which would be
great.

Author: pimterry

src/client/admin-client.ts

@@ -11,6 +11,7 @@ import { print } from 'graphql';
 import { DEFAULT_ADMIN_SERVER_PORT } from "../types";
 
 import { MaybePromise, RequireProps } from '../util/type-utils';
+import { isNode } from '../util/util';
 import { isErrorLike } from '../util/error';
 import { getDeferred } from '../util/promise';
 
@@ -21,7 +22,9 @@ import { SchemaIntrospector } from './schema-introspection';
 import { AdminQuery, getSingleSelectedFieldName } from './admin-query';
 import { MockttpOptions } from '../mockttp';
 
-const { fetch, Headers } = typeof globalThis.fetch === 'undefined' ? CrossFetch : globalThis;
+const { fetch, Headers } = isNode || typeof globalThis.fetch === 'undefined'
+    ? CrossFetch
+    : globalThis;
 
 export class ConnectionError extends TypedError { }