fix: no-op check to work with Safari and support of old browsers. (#582)

Verify the object that might be at `globalThis.browser` is not already
implementing the basic Web Extension APIs. This is symmetric with the
check to verify the poly fill is only included in extension contexts.

Don't use optional-chaining for these checks, since this needs to
continuing working in older browsers.

src/browser-polyfill.js

@@ -6,11 +6,11 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 "use strict";
 
-if (!globalThis.chrome?.runtime?.id) {
+if (!(globalThis.chrome && globalThis.chrome.runtime && globalThis.chrome.runtime.id)) {
   throw new Error("This script should only be loaded in a browser extension.");
 }
 
-if (typeof globalThis.browser === "undefined" || Object.getPrototypeOf(globalThis.browser) !== Object.prototype) {
+if (!(globalThis.browser && globalThis.browser.runtime && globalThis.browser.runtime.id)) {
   const CHROME_SEND_MESSAGE_CALLBACK_NO_RESPONSE_MESSAGE = "The message port closed before a response was received.";
 
   // Wrapping the bulk of this polyfill in a one-time-use function is a minor