feat(tazjin/rlox): Implement global variable definition

identifier_str might look a bit overengineered, but we want to reuse
this bit of code and it needs a reference to the token from which to
pick the identifier.

The problem with this is that the token would be owned by self, but
the function needs to mutate (the interner), so this implementation is
the most straightforward way of acquiring and working with an
immutable reference to the token before interning the identifier.

Change-Id: I618ce8f789cb59b3a9c5b79a13111ea6d00b2424
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2592
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

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

@@ -29,4 +29,7 @@ pub enum OpCode {
     // Built in operations
     OpPrint,
     OpPop,
+
+    // Variable definitions
+    OpDefineGlobal(usize),
 }