Nested region tags are stripped from imported C# code using `code-csharp`

**DocFX Version Used**: 2.21.1

**Steps to Reproduce**:
1. Create C# file with nested regions such as:
```csharp
#region OUTER
public class Foo{
#region INNER
// hello
#endregion
}
#endregion
```

2. Import the cshrap into markdown using the DFM syntax, selecting the outer region.
`[!code-csharp[Main](testfile.cs#OUTER)]`

**Expected Behavior**:
Contents of the `OUTER` region are displayed.
```csharp
public class Foo{
#region INNER
// hello
#endregion
}
```

**Actual Behavior**:
The `INNER` region tags (both start and end) are dropped.
```csharp
public class Foo{
//hello
}
```

Including the whole file, or including the range with line numbers works as expected (regions are not dropped).