Make Base.Cartesian public (#59459)
Some of the stuff in this module has slightly silly but extremely useful
API (e.g. the identification of `i` in variable names in `Base.@nexprs
10 i -> x_i = i`). Make it public.
As a point of reference, nothing more, `@nexprs` is already widely used
https://juliahub.com/ui/Search?type=symbols&q=@nexprs&t=macro
closes https://github.com/JuliaLang/julia/issues/58378
