narrow to `decimal-int-string` via cast

[staabm]

Feature request

PHPStan is detecting decimal-int-string for ConstantStringType in https://github.com/phpstan/phpstan-src/blob/7fe8b00282cff26f814cbddbec9906a1f4802301/src/Type/Constant/ConstantStringType.php#L399

we should narrow types when analyzing code the same way

https://phpstan.org/r/939b0b5f-4cfe-4aea-bfb5-3620b6cd1a2d