Fix Union Array type StaticTypeMapper to string

[TomasVotruba]

Closes #2627

[TomasVotruba]

@gnutix This one was tricky to resolve. I used new branch for easier work and so the traditional syntax was used. For the PHPStan syntax, you'd have to implement optional parameter to prefer that everywhere.

To the issue: The problem is that PHPStan and php-parser see this content in different ways:

int[]|string[]

For one it is array of union types, something like (int|string)[]
for the other it's union type of array, something like int[]|string[]

We'll need refactor this whole converting system, to handle deep nested conversion, and also come up with some simple testing idea, that will cover these type transformations.