feat: Add train/val split script and update gitignore

- Added 03_train_val_split.py to create deterministic train/validation splits from data.txt or fallback text
- Updated .gitignore to un-comment .vscode/ directory exclusion
- Changed data.txt pattern to *.txt for better file matching in gitignore
- Script handles UTF-8 text loading with newline normalization and writes train.txt/val.txt files
- Includes doctest examples and proper type hints

.gitignore

@@ -199,7 +199,7 @@ cython_debug/
 #   that can be found at https://github.com/github/gitignore/blob/main/Global/VisualStudioCode.gitignore
 #   and can be added to the global gitignore or merged into this file. However, if you prefer, 
 #   you could uncomment the following to ignore the entire vscode folder
-# .vscode/
+.vscode/
 
 # Ruff stuff:
 .ruff_cache/
@@ -216,4 +216,4 @@ __marimo__/
 .streamlit/secrets.toml
 
 # Data/Material that should not be synced
-data.txt
+*.txt