Add check for running JVM when adding callbacks

This commit adds a check to see if the JVM has already been started.
If True, then run the callback immediately instead of appending to the
global callbacks list. This aligns with the callback mechanism in
scyjava.

src/imagej/__init__.py

@@ -1282,9 +1282,13 @@ def when_imagej_starts(f) -> None:
         as its sole argument, and called as the final action of the
         init function before it returns or blocks.
     """
-    # Add function to the list of callbacks to invoke upon start_jvm().
-    global _init_callbacks
-    _init_callbacks.append(f)
+    if sj.jvm_started():
+        # JVM already started, invoke callback immediately.
+        f()
+    else:
+        # Add function to the list of callbacks to invoke upon start_jvm().
+        global _init_callbacks
+        _init_callbacks.append(f)
 
 
 def imagej_main():