Add GDScript .editorconfig rules

- Uniformize `.gd` unit test files indentation to tabs (where needed)

.editorconfig

@@ -21,3 +21,13 @@ indent_size = 4
 [*.{yml,yaml}]
 indent_style = space
 indent_size = 2
+
+# GDScript unit test files
+[*.gd]
+indent_style = tab
+indent_size = 4
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.out]
+insert_final_newline = true