set ipython accessor at _plot_area module level

It's not accessible from within the scope of the method for some reason, can just have it at module level scope instead, shouldn't be any caveats to this:

https://github.com/fastplotlib/fastplotlib/blob/68cf50b319d76196d4bed487128d36f1616706d1/fastplotlib/layouts/_plot_area.py#L676

