Add missing type for TextInput.readOnly in Typescript (#39281)

Summary:
The `readOnly` prop is available in the `TextInput` implementation (see [usage https://github.com/facebook/react-native/issues/1](https://github.com/facebook/react-native/blob/main/packages/react-native/Libraries/Components/TextInput/TextInput.js#L1646), [usage https://github.com/facebook/react-native/issues/2](https://github.com/facebook/react-native/blob/main/packages/react-native/Libraries/Components/TextInput/TextInput.js#L1671)), but the corresponding Typescript type is currently missing. Those who use Typescript get an invalid prop error when using the `readOnly` prop on `TextInput`.

Having this prop type included in TypeScript would fix the bug above and also would be beneficial for consistency and comprehensive type support.

Notably, this prop is also included in the Flow types [here](https://github.com/facebook/react-native/blob/main/packages/react-native/Libraries/Components/TextInput/TextInput.flow.js#L814).

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[GENERAL] [FIXED] - Add missing type for TextInput.readOnly in Typescript

Pull Request resolved: https://github.com/facebook/react-native/pull/39281

Test Plan: N/A

Reviewed By: rshest

Differential Revision: D48955477

Pulled By: NickGerleman

fbshipit-source-id: 9d3431351b8ad65a4d2fc57939b98c167dc70cee

packages/react-native/Libraries/Components/TextInput/TextInput.d.ts

@@ -814,6 +814,11 @@ export interface TextInputProps
    */
   placeholderTextColor?: ColorValue | undefined;
 
+  /**
+   * If `true`, text is not editable. The default value is `false`.
+   */
+  readOnly?: boolean | undefined;
+
   /**
    * enum('default', 'go', 'google', 'join', 'next', 'route', 'search', 'send', 'yahoo', 'done', 'emergency-call')
    * Determines how the return key should look.