feat: error reporting for global reference usage

src/error_tests.zig

@@ -27,6 +27,19 @@ test "compiler: global variable shadowing" {
     );
 }
 
+test "compiler: invalid divert target" {
+    try testEqual(
+        \\-> foo
+        \\
+        \\VAR foo = 123
+    ,
+        \\<STDIN>:1:4: error: invalid divert target
+        \\1    | -> foo
+        \\     |    ^
+        \\
+    );
+}
+
 fn testEqual(source_bytes: [:0]const u8, expected_error: []const u8) !void {
     const gpa = std.testing.allocator;
     var allocating = std.io.Writer.Allocating.init(gpa);