restructure: move mod files into plugin/ directory

plugin/info.json

@@ -0,0 +1,9 @@
+{
+  "name": "SignalExporter",
+  "version": "1.0.0",
+  "title": "Signal Exporter",
+  "author": "Expert Engineer",
+  "factorio_version": "1.1",
+  "dependencies": ["base >= 1.1.0"],
+  "description": "Provides a custom combinator that reads both connected circuit and logistic networks, writing the data to a JSON file."
+}