Allow resizing of vectors at any position

[Tortar]

Resizing of vectors is currently supported only at the end (but after #57313 is merged, it will be possible to resize also at the start).

I think that having in the API something along the lines of resizeat!(a::Vector, n::Int, pos::Int) where pos indicates the position of the resizing makes sense (and similar for BitVector).

There is already an internal function to allow that for a Vector, i.e. _growat!, which has the nice property that the more you are near the extrema the more the resizing is faster.

[jakobnissen]

Good idea! If this is implemented, then it would be nice to also define insert!(::Vector, ::UnitRange, xs) to be a resize! followed by a copyto!.