Refactor GDScript/C# script templates logic to be editor-only

Not a full refactor as it still goes through ScriptLanguage so it's hacky,
but at least it can now compile without this.

modules/mono/editor/script_templates/EditorPlugin/plugin.cs

@@ -0,0 +1,20 @@
+// meta-description: Basic plugin template
+
+#if TOOLS
+using _BINDINGS_NAMESPACE_;
+using System;
+
+[Tool]
+public partial class _CLASS_ : _BASE_
+{
+    public override void _EnterTree()
+    {
+        // Initialization of the plugin goes here.
+    }
+
+    public override void _ExitTree()
+    {
+        // Clean-up of the plugin goes here.
+    }
+}
+#endif