feat: implement command execution from user input

Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "jshell"
+version = "0.1.0"

src/main.rs

@@ -1,3 +1,13 @@
+use std::{io::stdin, process::Command};
+
 fn main(){
-    println!("Hello, world!");
+    let mut input = String::new();
+    stdin().read_line(&mut input).unwrap();
+
+    // read_line leaves a trailing newline, which trim removes
+    let command = input.trim(); 
+
+    Command::new(command)
+        .spawn()
+        .unwrap();
 }