Adds in-game debug menu addon

Adds an in-game debug menu that displays performance metrics (FPS, frame times) and hardware/software information.

The menu can be toggled using the F3 key (or a custom input binding). It has different display styles, ranging from a compact FPS display to a detailed view with graphs and system information.

addons/debug_menu/plugin.gd

@@ -0,0 +1,29 @@
+@tool
+extends EditorPlugin
+
+func _enter_tree() -> void:
+	add_autoload_singleton("DebugMenu", "res://addons/debug_menu/debug_menu.tscn")
+
+	# FIXME: This appears to do nothing.
+#	if not ProjectSettings.has_setting("application/config/version"):
+#		ProjectSettings.set_setting("application/config/version", "1.0.0")
+#
+#	ProjectSettings.set_initial_value("application/config/version", "1.0.0")
+#	ProjectSettings.add_property_info({
+#		name = "application/config/version",
+#		type = TYPE_STRING,
+#	})
+#
+#	if not InputMap.has_action("cycle_debug_menu"):
+#		InputMap.add_action("cycle_debug_menu")
+#		var event := InputEventKey.new()
+#		event.keycode = KEY_F3
+#		InputMap.action_add_event("cycle_debug_menu", event)
+#
+#	ProjectSettings.save()
+
+
+func _exit_tree() -> void:
+	remove_autoload_singleton("DebugMenu")
+	# Don't remove the project setting's value and input map action,
+	# as the plugin may be re-enabled in the future.