## Add `bySemanticsIdentifier` finder for finding by identifier
### Description
This pull request introduces a new finder, `CommonFinders.bySemanticsIdentifier`, to the Flutter testing framework. This finder allows developers to locate `Semantics` widgets based on their `identifier` property, enhancing the precision and flexibility of widget tests.
### Motivation
Establish a consistent and reliable method for locating elements in integration and end-to-end (e2e) tests. Unlike `label` or `key`, which may carry functional significance within the application, the `identifier` is purely declarative and does not impact functionality. Utilizing the `identifier` for finding semantics widgets ensures that tests can target specific elements without interfering with the app's behavior, thereby enhancing test reliability, maintainability, and reusability across testing frameworks.
### Changes
- **semantics.dart**
- Updated documentation to mention that `identifier` can be matched using `CommonFinders.bySemanticsIdentifier`.
- **finders.dart**
- Added the `bySemanticsIdentifier` method to `CommonFinders`.
- Supports both exact string matches and regular expression patterns.
- Includes error handling to ensure semantics are enabled during tests.
- **finders_test.dart**
- Added tests to verify that `bySemanticsIdentifier` correctly finds widgets by exact identifier and regex patterns.
- Ensures that the finder behaves as expected when semantics are not enabled.
### Usage
Developers can use the new finder in their tests as follows:
```dart
// Exact match
expect(find.bySemanticsIdentifier('Back'), findsOneWidget);
// Regular expression match
expect(find.bySemanticsIdentifier(RegExp(r'^item-')), findsNWidgets(2));
```
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