Detect unused goto label

### Feature request

I'd like PHPStan to catch a literal string on its own
e.g.
```
http:
```
https://phpstan.org/r/75263a21-1b5f-4339-88b6-0efff221d973

Syntax check does not detect it.
It causes absolutely no effects during execution.


### Did PHPStan help you today? Did it make you happy in any way?

_No response_