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/gdscript/editor/script_templates/EditorPlugin/plugin.gd

@@ -0,0 +1,13 @@
+# meta-description: Basic plugin template
+@tool
+extends EditorPlugin
+
+
+func _enter_tree() -> void:
+	# Initialization of the plugin goes here.
+	pass
+
+
+func _exit_tree() -> void:
+	# Clean-up of the plugin goes here.
+	pass