[API Proposal]: Add GetCharacterIndexFromPoint method to TextBlock

### Background and motivation

In my app I need to get the character index from the mouse position. Even though I don't need the capabilities of a `TextBox`, because `TextBlock` does not have a method like [GetCharacterIndexFromPoint](https://learn.microsoft.com/en-us/dotnet/api/system.windows.controls.textbox.getcharacterindexfrompoint), I need to use the TextBox control instead. The issue is my app creates _lots of_ TextBoxes (which could have been TextBlocks if a TextBlock supported the aforementioned method) and a TextBox is more resource hungry than a TextBlock. So I'd like to avoid paying the price of a TextBox unnecessarily.

### API Proposal

```csharp
public int GetCharacterIndexFromPoint (System.Windows.Point point, bool snapToText);
```

### API Usage

```csharp
TextBlock textBlock = new();
int characterPosition = textBlock.GetCharacterIndexFromPoint(Mouse.GetPosition(textBlock), false);
```

### Alternative Designs

_No response_

### Risks

_No response_