Add IS_STORYBOOK global variable

This is to let user code detect if it is running in Storybook.

docs/faq.md

@@ -349,29 +349,4 @@ Applying this small change will enable you to add syntax highlight for SCSS or a
 
 ### How can my code detect if it is running in Storybook?
 
-You can do this by checking the value of `process.env.STORYBOOK`, which will
-equal `'true'` when running in Storybook. Be careful — `process` may be
-undefined when your code runs outside of Storybook.
-
-```javascript
-export function isRunningInStorybook() {
-  try {
-    if (process.env.STORYBOOK) return true;
-  } catch {
-    // A ReferenceError will be thrown if process is undefined
-  }
-
-  return false;
-}
-```
-
-This works because Babel replaces `process.env.STORYBOOK` with the value of the
-`STORYBOOK` environment variable. Because this is done through a Babel plugin,
-the following will **NOT** work:
-
-```javascript
-export function isRunningInStorybook() {
-  return typeof process?.env?.STORYBOOK !== 'undefined';
-  // ReferenceError: process is not defined
-}
-```
+You can do this by checking for the `IS_STORYBOOK` global variable, which will equal `true` when running in Storybook.

lib/core-client/src/preview/start.test.ts

@@ -1,3 +1,4 @@
+/* global window */
 import Events from '@storybook/core-events';
 
 import {
@@ -357,6 +358,8 @@ describe('start', () => {
         }),
         undefined
       );
+
+      expect((window as any).IS_STORYBOOK).toBe(true);
     });
 
     it('supports forceRerender()', async () => {

lib/core-client/src/preview/start.ts

@@ -34,6 +34,11 @@ export function start<TFramework extends AnyFramework>(
     render?: ArgsStoryFn<TFramework>;
   } = {}
 ) {
+  if (globalWindow) {
+    // To enable user code to detect if it is running in Storybook
+    globalWindow.IS_STORYBOOK = true;
+  }
+
   if (FEATURES?.storyStoreV7) {
     return {
       forceReRender: removedApi('forceReRender'),