Convert T4 to StringBuilder.Append($$""" """);

tt2rawstring.cs

using System.Text.RegularExpressions;
using System.Windows;

class Program
{
    [STAThread]
    static void Main()
    {
        var x = Clipboard.GetText();

        x = Reg.A().Replace(x, m => "{{" + m.Groups[1].Value + "}}");
        x = Reg.C().Replace(x, """"
                    s.Append($$$$"""
            """");
        x = Reg.B().Replace(x, """"

            """);
            """");

        Clipboard.SetText(""""
                        
                public string TransformText()
                {
                    var s = new StringBuilder();

                    s.Append($$"""
            
            """"
            + x +
            """"

            """);

                return s.ToString();
            }

            """");
    }
}

static partial class Reg
{
    [GeneratedRegex("""
        \<\#\=(.*?)\#\>
        """, RegexOptions.Multiline)]
    public static partial Regex A();

    [GeneratedRegex("""
        ^\<\#
        """, RegexOptions.Multiline)]
    public static partial Regex B();

    [GeneratedRegex("""
        ^\#\>
        """, RegexOptions.Multiline)]
    public static partial Regex C();
}