feat(tazjin/rlox): Add global variable support in interpreter

Change-Id: I4134cf78dc3934a517ad0c848ae1d3729abaf882
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2297
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/rlox/src/errors.rs

@@ -7,6 +7,8 @@ pub enum ErrorKind {
     ExpectedSemicolon,
     ExpectedVariableName,
     TypeError(String),
+    UndefinedVariable(String),
+    InternalError(String),
 }
 
 #[derive(Debug)]