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/NonDarwinLogging.swift

@@ -346,6 +346,11 @@ package struct NonDarwinLogger: Sendable {
       // sourcekitd response). 10.000 characters was chosen because it seems to fit the result of most sourcekitd
       // responses that are not generated interface or global completion results (which are a lot bigger).
       var message = message().value.string(for: self.privacyLevel)
+
+      // 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)
+
       if message.utf8.count > 10_000 {
         // Check for UTF-8 byte length first because that's faster since it doesn't need to count UTF-8 characters.
         // Truncate using `.prefix` to avoid cutting of in the middle of a UTF-8 multi-byte character.