feat(tazjin/rlox): Parse if statements

Change-Id: I2352d75a3f02d65a5a2d04fb2cc4daa50f11ca1e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2321
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/rlox/src/interpreter.rs

@@ -127,6 +127,7 @@ impl Interpreter {
             }
             Statement::Var(var) => return self.interpret_var(var),
             Statement::Block(block) => return self.interpret_block(block),
+            Statement::If(_) => unimplemented!(),
         }
 
         Ok(())