- Remove unused filetypes.rs and walk.rs modules

- Introduce `index.rs` for file indexing using SQLite
- Expand configuration options in `config.rs`, including `excluded_files`
- Update dependencies in `Cargo.toml` to include SQLite, hashing, and regex libraries

README.md

@@ -0,0 +1,4 @@
+## Credit
+
+Portions of `walk.rs` are adapted from [fd](https://github.com/sharkdp/fd) by the fd developers (MIT License).  
+See [`walk.rs`](src/walk.rs) for modification details.