unknown expression: `x=?y`
reported here: https://discord.com/channels/215267007245975552/1131687395088142357/1131687395088142357

minimal reproduction:
```
echo 'local x=?"hi"' | shrinko8 - - --minify
Compilation errors:
-:1:10: unknown expression
```

(note that `echo 'local x=print"hi"' | shrinko8 - - --minify` works as expected, producing `local l=print"hi"`)