Check for explicitly set logger config before overwriting it (#70)

This change allows users to utilize the `log4j2.configurationFile`
system property for configuring the logging context to their liking.

This property is read by log4j2 to find the configuration file it should
load during initialization. Previously, setting this property only
affected logging output during the boot stage (anything before launching
Minecraft itself) due to the unconditional logger context reconfiguration.

Tying this logic to the already existing system property used by log4j2
seemed the most logical, considering that this property is the only
approachable way for specifying an alternative configuration in the
first place.

loader/src/main/java/net/neoforged/fml/loading/targets/CommonLaunchHandler.java

@@ -57,17 +57,26 @@ public void configureTransformationClassLoader(final ITransformingClassLoaderBui
     }
 
     protected String[] preLaunch(String[] arguments, ModuleLayer layer) {
+        // do not overwrite the logging configuration if the user explicitly set another one
+        if (System.getProperty("log4j2.configurationFile") == null) {
+            overwriteLoggingConfiguration(layer);
+        }
+
+        return arguments;
+    }
+
+    /**
+     * Forces the log4j2 logging context to use the configuration shipped with fml_loader.
+     */
+    private void overwriteLoggingConfiguration(final ModuleLayer layer) {
         URI uri;
         try (var reader = layer.configuration().findModule("fml_loader").orElseThrow().reference().open()) {
             uri = reader.find("log4j2.xml").orElseThrow();
         } catch (IOException e) {
             throw new RuntimeException(e);
         }
 
-        // Force the log4j2 configuration to be loaded from fmlloader
         Configurator.reconfigure(ConfigurationFactory.getInstance().getConfiguration(LoggerContext.getContext(), ConfigurationSource.fromUri(uri)));
-
-        return arguments;
     }
 
     protected final String[] getLegacyClasspath() {