support syntax highlighting for code documentation comments

[Meir017]

Summary

In many libraries authors add code sample inside the <code> documentation comment, it would be helpful to have syntax highlighting for these code comments

Background and Motivation

This type of syntax-highlighting already exists in the form of adding a // lang=C#-test or annotation an argument with [StringSyntax("C#-test")]

however, this doesn't work

// lang=C#-test
var code = """
using System;
using System.Text;
var builder = new StringBuilder();
test.AppendLine(""Hello, World!"");
Console.WriteLine(builder.ToString());
""";

/// <code lang="C#-test">
/// using System;
/// using System.Text;
/// var builder = new StringBuilder();
/// test.AppendLine("Hello, World!");
/// Console.WriteLine(builder.ToString());
/// </code>
void Test() { }

from vscode:

[sharwell]

Changing from Feature Request to Design Debt, since this feature was originally implemented in #35667 and the more recent LSP work failed to meet parity.

[CyrusNajmabadi]

@sharwell we added support for c# syntax highlighting within a doc comment codeblock in VS?

[sharwell]

@CyrusNajmabadi Sorry, I added the above comment after seeing #75215. There are two separate reports here: a request for syntax highlighting, and an observation that the comment renders as inline text instead of a code block. The part implemented previously is just the code block portion.