add file logging

SimpleTGBot/Logging/FileSink.cs

@@ -0,0 +1,22 @@
+namespace SimpleTGBot.Logging;
+
+internal class FileSink : ILogSink
+{
+    StreamWriter file;
+
+    public FileSink(string filename)
+    {
+        file = new StreamWriter(filename, true);
+    }
+
+    public void Log(DateTime time, LogLevel level, string message)
+    {
+        foreach (string line in message.Split(Environment.NewLine))
+            file.WriteLine($"({time:u}) [{level.GetName()}] {line}");
+    }
+
+    public void Dispose()
+    {
+        file.Dispose();
+    }
+}