Add LECO training script and associated tests

- Implemented `sdxl_train_leco.py` for training with LECO prompts, including argument parsing, model setup, training loop, and weight saving functionality.
- Created unit tests for `load_prompt_settings` in `test_leco_train_util.py` to validate loading of prompt configurations in both original and slider formats.
- Added basic syntax tests for `train_leco.py` and `sdxl_train_leco.py` to ensure modules are importable.

tests/test_train_leco.py

@@ -0,0 +1,5 @@
+import train_leco
+
+
+def test_syntax():
+    assert train_leco is not None