feat: error message for unknown global variables

src/error_tests.zig

@@ -1,6 +1,18 @@
 const std = @import("std");
 const Compilation = @import("compile.zig").Compilation;
 
+test "compiler: unknown global variable" {
+    try testEqual(
+        \\{a}
+    ,
+        \\<STDIN>:1:2: error: unknown global variable
+        \\1    | {a}
+        \\     |  ^
+        \\
+        ,
+    );
+}
+
 test "compiler: global variable shadowing" {
     try testEqual(
         \\VAR a = 0