Index a `CartesianIndex` with `begin`/`end` (#57985)
Currently, it is possible to index into a `CartesianIndex` using an
`Int` index.
```julia
julia> I = CartesianIndex(4,3)
CartesianIndex(4, 3)
julia> I[1]
4
julia> I[2]
3
```
This PR adds the ability to use `begin`/`end` in the indexing:
```julia
julia> I[begin]
4
julia> I[end]
3
```
The advantage of this (particularly indexing with `end`) is that one
doesn't need to know the number of dimensions. This makes writing
generic code that accepts either a vector or a matrix easier.
