feat: add support for let statements

tests/let_stmt.src

@@ -0,0 +1,11 @@
+fn main() -> i32 {
+    let a = 10;
+    let b: i32 = 20;
+    let c = a + b;
+    {
+        // Shadow 'a' and 'c' in a new scope
+        let a = 5;
+        let c = a + b;
+    }
+    return c; // Should return 30, not 25, because the inner 'c' is dropped
+}