Negative literals are borked

`print -5;` says:

```
LHS missing for `-` at 0:6
Expected expression, found: `)` at 0:8
Parsing failed, see errors above.
```

The issue is in the parser's handling of unary minus. Look at the parser module, likely in src/parser.rs, for the expression parsing logic. The error messages indicate it expects a left-hand side for the '-' operator. Check how the parser distinguishes between binary subtraction and unary negation. Run the existing tests to see if there are any for negative literals, and add a test case for `print -5;` to verify the fix.

索引モデルが issue の本文から書いたものです。