Avoiding `instanceof Request`, etc.

In Node.js 18.6, this piece of code doesn't work:

import { fetch } from "undici";

fetch(new Request("https://example.com"));

Error:

TypeError: Failed to parse URL from [object Request]
    at fetch (<>/node_modules/undici/index.js:109:13) {
  [cause]: TypeError [ERR_INVALID_URL]: Invalid URL
      at new NodeError (node:internal/errors:405:5)
      at new URL (node:internal/url:743:13)
      at new Request (<>/node_modules/undici/lib/fetch/request.js:86:21)
      at fetch (<>node_modules/undici/lib/fetch/index.js:137:21)
      at fetch (<>/node_modules/undici/index.js:107:20)
      at file://<>/test.js:3:1
      at ModuleJob.run (node:internal/modules/esm/module_job:192:25)
      at async DefaultModuleLoader.import (node:internal/modules/esm/loader:246:24)
      at async loadESM (node:internal/process/esm_loader:40:7)
      at async handleMainPromise (node:internal/modules/run_main:66:12) {
    input: '[object Request]',
    code: 'ERR_INVALID_URL'
  }
}

This is because instanceof Request fails due to the global Request and the Undici Request not being the same.

https://github.com/nodejs/undici/blob/7153a1c78d51840bbe16576ce353e481c3934701/lib/fetch/request.js#L854

While many libraries accept a fetch argument (like ky), not many libraries accept another set of globals (Request, Response, Headers). Next to that, global.Request is a read-only property in Node.js. I think that instead of using instanceof Undici should be using another type of check for these. (e.g. request[Symbol.toStringTag] === 'Request').

I'd like to collect thoughts and then I'll create a PR.

This would also fix https://github.com/cloudflare/miniflare/issues/454

Start in lib/fetch/request.js around the instanceof check referenced in the issue, then reproduce the Node.js 18.6 example using the global Request and Undici's fetch. Review the comment discussion for the expected cross-global behavior. Done means fetch accepts the global Request without treating it as a URL string, while preserving normal Request handling.