Validate names for invalid for C# namespaces in cmdpal (#39401)

## Summary of the Pull Request

The Command Palette allows users to create extensions with dashes in their names, which is invalid for C# namespaces. This causes projects to fail during build because the name cannot be used as a valid namespace.

## Changes
- Updated the validation regex in `NewExtensionForm.cs` to only allow valid C# identifiers
  - Now only allows names starting with a letter or underscore, followed by letters, numbers, or underscores
  - Explicitly prevents dashes, spaces, and other special characters
- Improved the error message to clearly explain the requirements for valid C# identifiers

## Before
Previously, the input only validated that no spaces were used:
```csharp
"regex": "^[^\\s]+$"
```

## After
Now the input properly validates for C# namespace compatibility:
```csharp
"regex": "^[a-zA-Z_][a-zA-Z0-9_]*$"
```

This ensures that users cannot create extension projects with names that would fail to build.![image](https://github.com/user-attachments/assets/c2fb5108-b32b-4411-84a8-45ef0c621372)

## PR Checklist

- [ ] **Closes:** https://github.com/microsoft/PowerToys/issues/38522

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/NewExtensionForm.cs

@@ -55,7 +55,7 @@ internal sealed partial class NewExtensionForm : NewExtensionFormBase
             "errorMessage": {{FormatJsonString(Properties.Resources.builtin_create_extension_name_required)}},
             "id": "ExtensionName",
             "placeholder": "ExtensionName",
-            "regex": "^[^\\s]+$"
+            "regex": "^[a-zA-Z_][a-zA-Z0-9_]*$"
         },
         {
             "type": "TextBlock",