feat(tazjin/rlox): Parse variable assignment & access

Change-Id: I9894d76716d739e85a4757d9e658f884228e7f52
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2290
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/errors.rs

@@ -5,6 +5,7 @@ pub enum ErrorKind {
     UnmatchedParens,
     ExpectedExpression(String),
     ExpectedSemicolon,
+    ExpectedVariableName,
     TypeError(String),
 }