feat(tazjin/rlox): Implement global variable access

This also includes a fix for an issue where the identifiers of
variables were pushed onto the stack, which is incorrect.

Change-Id: Id89b388268efad295f29978d767aa4b33c4ded14
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2594
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/bytecode/opcode.rs

@@ -30,6 +30,7 @@ pub enum OpCode {
     OpPrint,
     OpPop,
 
-    // Variable definitions
+    // Variable management
     OpDefineGlobal(usize),
+    OpGetGlobal(usize),
 }