Log to stderr as well as the log file, on non-Darwin platforms

This is consistent with other LSP servers, and makes debugging
sourcekit-lsp easier.

Author: Wilfred

Sources/SKLogging/SetGlobalLogFileHandler.swift

@@ -116,6 +116,10 @@ private func logToFile(message: String, logDirectory: URL, logFileMaxBytes: Int,
 private func setUpGlobalLogFileHandlerImpl(logFileDirectory: URL, logFileMaxBytes: Int, logRotateCount: Int) {
   logHandler = { @LogHandlerActor message in
     do {
+      // In addition to writing to the log file, also log to stderr, so LSP output is visible in the editor
+      // (e.g. VS Code's Output panel).
+      fputs(message + "\n", stderr)
+      
       try logToFile(
         message: message,
         logDirectory: logFileDirectory,