Non-ASCII identifier syntax highlighting

The following code [compiles and run](https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA0AXEUCuA7AHwAEAmARgFgAoIgZgAJT6Bhagb2vq8bKR4AZ6wAIZQAFAEp6Aemn1AKOSB4P87cOVbhsYB2egCIRUXQG4VXAL7VTMuUV4D6AMwBvTyVdt9bggDo5+/JCRffwAxIL9+MMlrekAMcno4ehgEAAcYMAwYABN6YIDw0ILIvgwIegALYQA3GHoAZ2EAW1rIABtoemE6oVF6DoBZYQBLPCs1TW4iHV0MBAxjKwsqdxISMasNWwBOMWdXCQkjGPjE5LSM7McXelKK6tqG5vo2jq6eqD6PwZGNybIdvKBPJhYFISSHY4JJKpdKZHKAoogiJhG5lSo1epNFoQdofN4GT70b6jdQTHg7Azgo6yBTKUnmahmIA==), but the syntax highlighting in GitHub and VSCode do not identify escaped or unescaped non-ascii identifiers properly:
```c#
using System;
public class C
{
static string bar() // ✔️
{
return "bar";
}

// static string föö()
static string \u0066\u00F6\u00F6() // ✘ - expected \u0066\u00F6\u00F6 to have same color as bar or Main
{
return "txt";
}

public static void Main()
{
Console.WriteLine(föö()); // ✘ - expected föö to have same color as bar or Main
Console.WriteLine(\u0066\u00F6\u00F6()); // ✘ - expected \u0066\u00F6\u00F6 to have same color as bar or Main
Console.WriteLine(bar()); // ✔️
}
}
```

VSCode is at least highlighting `föö()` properly (GitHub linguist probably needs an update to sync with the latest grammar update which eventually consumes this repo).