fix: export TextButtonSize enum from design-system-react-native (#848)

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## **Description**

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also include relevant motivation and context. Have in mind the following
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1. What is the reason for the change?
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1. **Reason for the change:** The `TextButtonSize` enum was not being
exported from the root `design-system-react-native` package, making it
inaccessible to consumers. This was reported by a user (Elliot in
Slack).
2. **Improvement/Solution:** This PR updates
`packages/design-system-react-native/src/components/index.ts` to export
`TextButtonSize`, ensuring it is available when importing from the
package's root.

## **Related issues**

Fixes: N/A

## **Manual testing steps**

1. In a React Native project consuming
`@metamask/design-system-react-native`, attempt to import
`TextButtonSize`:
    ```typescript
import { TextButtonSize } from '@metamask/design-system-react-native';
    ```
2. Verify that `TextButtonSize` is successfully imported and can be used
(e.g., `TextButtonSize.Lg`).

## **Screenshots/Recordings**

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### **Before**

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### **After**

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app, test code being changed).
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---
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Thread](https://consensys.slack.com/archives/C0354T27M5M/p1761343338972079?thread_ts=1761343338.972079&cid=C0354T27M5M)

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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Expose `TextButtonSize` by exporting it from
`packages/design-system-react-native/src/components/index.ts`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
59e8d5b. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Co-authored-by: Cursor Agent <[email protected]>

Author: georgewrmarshall

packages/design-system-react-native/src/components/index.ts

@@ -87,7 +87,7 @@ export type { JazziconProps } from './temp-components/Jazzicon';
 export { Maskicon } from './temp-components/Maskicon';
 export type { MaskiconProps } from './temp-components/Maskicon';
 
-export { TextButton } from './TextButton';
+export { TextButton, TextButtonSize } from './TextButton';
 export type { TextButtonProps } from './TextButton';
 
 export {