Relax return type of String#byte_index to match offset

[HertzDevil]

Like #10972, but for String#byte_index.

[straight-shoota]

I know we did this before, but it's not the ideal solution. I think ideally we should restrict the return type and offset both to Int32. If offset can take a different type, the return type would be a union type. And there's really no good reason for that. An index is always Int32 in Crystal's stdlib.

It was okay to revert back in #10972 to fix a regression. Now these restrictions on #byte_index have been in place since Crystal 0.35 (introduced #8447), so it's not really a quick regression fix anymore.

I think the more reasonable action would be to add Int32 restriction on the offset parameter. This would document the implict restriction that has been observed for the past four years.