feat(tazjin/rlox): Implement mutable variable assignment

Change-Id: I56db10a5ac26958ae27a8d4c8fa7b8f8285bc7e1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2299
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/errors.rs

@@ -9,6 +9,7 @@ pub enum ErrorKind {
     TypeError(String),
     UndefinedVariable(String),
     InternalError(String),
+    InvalidAssignmentTarget(String),
 }
 
 #[derive(Debug)]