numeric-string as array-key is int|string and not int only

# Bug report

If there is a ```numeric``` type, and we cast it into a string, then it should be ```numeric&string``` or at least ```string``` but not int as in the next example.

### Code snippet that reproduces the problem

https://phpstan.org/r/d13c93de-adae-4900-a9b9-b9331c13e992

### Expected output

no error