Refactor: Migrate to clap for CLI argument parsing

The project now uses the `clap` crate for handling command-line
arguments, replacing the previous manual parsing approach. This includes
adding `clap` as a dependency and creating a `Cli` struct to define the
command-line interface. The `main` function has been updated to parse
these arguments and handle file input or direct script evaluation.

src/lib.rs

@@ -0,0 +1,4 @@
+pub mod ast;
+
+#[cfg(test)]
+mod integration_test;