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 #58378

Author: LilithHafner

base/public.jl

@@ -2,6 +2,7 @@
 
 public
 # Modules
+    Cartesian,
     Checked,
     Filesystem,
     Order,