[wpiutil] Add JNI access to C++ stderr (#4381)

This is useful in some debugging scenarios.  System.err is separately buffered, so when e.g. debugging test cases it doesn't interleave correctly with the C++ stdout/stderr logging. Even using flush() doesn't seem to help, I think because Gradle does its own buffering.

wpiutil/src/main/native/cpp/jni/WPIUtilJNI.cpp

@@ -5,6 +5,7 @@
 #include <jni.h>
 
 #include "edu_wpi_first_util_WPIUtilJNI.h"
+#include "fmt/format.h"
 #include "wpi/Synchronization.h"
 #include "wpi/jni_util.h"
 #include "wpi/timestamp.h"
@@ -41,6 +42,18 @@ JNIEXPORT void JNICALL JNI_OnUnload(JavaVM* vm, void* reserved) {
   interruptedEx.free(env);
 }
 
+/*
+ * Class:     edu_wpi_first_util_WPIUtilJNI
+ * Method:    writeStderr
+ * Signature: (Ljava/lang/String;)V
+ */
+JNIEXPORT void JNICALL
+Java_edu_wpi_first_util_WPIUtilJNI_writeStderr
+  (JNIEnv* env, jclass, jstring str)
+{
+  fmt::print(stderr, "{}", JStringRef{env, str});
+}
+
 /*
  * Class:     edu_wpi_first_util_WPIUtilJNI
  * Method:    enableMockTime