fix: Guard against process not being defined (#911)

Webpack 5 doesn't shim Node's process object by default anymore. Only instances of process.env.NODE_ENV are replaced statically. This means that unguarded checks of process.env will crash in browser environments (such as Karma).

src/index.js

@@ -5,7 +5,7 @@ import {cleanup} from './pure'
 // this ensures that tests run in isolation from each other
 // if you don't like this then either import the `pure` module
 // or set the RTL_SKIP_AUTO_CLEANUP env variable to 'true'.
-if (!process.env.RTL_SKIP_AUTO_CLEANUP) {
+if (typeof process === "undefined" || !process.env?.RTL_SKIP_AUTO_CLEANUP) {
   // ignore teardown() in code coverage because Jest does not support it
   /* istanbul ignore else */
   if (typeof afterEach === 'function') {