Feature request

There are string functions that we can send numeric string into, that will often return numeric string.
Examples:

• trim, rtrim, ltrim on a numeric-string will either be empty, or a numeric string. But if second parameter is not dynamic and not a number, we can know for sure that it'll return a numeric-string as well. Example: ltrim($number, '-') will be numeric, and is a nice way of getting the absolute value of a numeric-string.

Given the proper guidance, and assuming this would be accepted, I'd be happy to contribute and open a pull request on phpstan-src.

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