feat(tests): add tests

added tests for the struct displays, eval function, lexer, and parser

justfile

@@ -6,6 +6,11 @@ _default:
 run *args:
     cargo run {{ args }}
 
+# Run the tests
+[group('dev')]
+test *args:
+    cargo test {{ args }}
+
 # Clean up previous builds
 [group('build')]
 clean: