added repl, eof token, and a parser with a AST producer

src/frontend.rs

@@ -0,0 +1,28 @@
+use std::process;
+
+use crate::parser::{self, Parser};
+
+pub fn repl() {
+    // Read-Eval-Print Loop
+    use std::io::{self, Write};
+
+    println!("Welcome to the Riv Lang REPL! Type 'exit' to quit.");
+
+    loop {
+        print!("> ");
+        io::stdout().flush().unwrap();
+
+        let mut input = String::new();
+        io::stdin().read_line(&mut input).unwrap();
+
+        let input = input.trim().to_string();
+
+        if input == "exit" {
+            process::exit(0);
+        }
+
+        let program = Parser::new(input).produce_ast();
+
+        println!("{:#?}", program);
+    }
+}