[Storage] Change StorageSharedKeyCredential browser stub to class (Az…

…ure#8568)

This fixes Azure#8499.

The issue was that in StorageClient.ts we have the following check
and assume that in browser scenario isNode would be false which
would short-circuit the evaluation of factory instanceof StorageSharedKeyCredential. So we added an undefined const for
browser stub of StorageSharedKeyCredential.

```typescript
      if (
        (isNode && factory instanceof StorageSharedKeyCredential) ||
        factory instanceof AnonymousCredential ||
        isTokenCredential(factory)
      ) {
        this.credential = factory;
      }
```

However, in an Electron + Angular application, isNode is true but
browser files are used. So the condition after && is evaluated and
caused an error:

```
TypeError: Right-hand side of 'instanceof' is not an object
at DataLakeServiceClient.StorageClient (StorageClient.js:35)
at new DataLakeServiceClient (DataLakeServiceClient.js:39)
```

This fix changes the stub of StorageSharedKeyCredential to an empty
class instead so the condition will not throw but be evaluated to
false because this credential type is not supported in browser.

sdk/storage/storage-blob/src/credentials/StorageSharedKeyCredential.browser.ts

@@ -1,4 +1,4 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
-export const StorageSharedKeyCredential = undefined;
+export class StorageSharedKeyCredential {}

sdk/storage/storage-file-datalake/src/credentials/StorageSharedKeyCredential.browser.ts

@@ -1,4 +1,4 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
-export const StorageSharedKeyCredential = undefined;
+export class StorageSharedKeyCredential {}

sdk/storage/storage-file-share/src/credentials/StorageSharedKeyCredential.browser.ts

@@ -1,4 +1,4 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
-export const StorageSharedKeyCredential = undefined;
+export class StorageSharedKeyCredential {}

sdk/storage/storage-queue/src/credentials/StorageSharedKeyCredential.browser.ts

@@ -1,4 +1,4 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
-export const StorageSharedKeyCredential = undefined;
+export class StorageSharedKeyCredential {}