[BUG] Missing shared implementation of SelectAllTextBehavior

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Did you read the "Reporting a bug" section on Contributing file?

- [X] I have read the "Reporting a bug" section on Contributing file: https://github.com/CommunityToolkit/Maui/blob/main/CONTRIBUTING.md#reporting-a-bug

### Current Behavior

I'm developing an application which consists of several projects, most of them targes both net8 and net8-android, net8 to allow testing of business logic using xUnit testing projects targeting net8.

But when I want to use the SelectAllTextBehavior I can't use it, because it does not have an empty implementation for net8.

I'm using xaml, so as far as I know, I can't use any kind of conditional compiling.

### Expected Behavior

An empty implementation of SelectAllTextBehavior for net8.0

### Steps To Reproduce

Create a library targeting net8.0 and net8.0-android, try to add SelectAllTextBehavior using xaml.

### Link to public reproduction project repository

https://github.com/jenscski/mct-missing-shared-SelectAllTextBehavior

### Environment

```markdown
- .NET MAUI CommunityToolkit:7.0.0
- OS: Windows 11
- .NET MAUI: 8.0.3
```

### Anything else?

_No response_