Enable option of minimal install without dask and dask-image

There have been requests (e.g. @bmorris) to have a way to install reproject without pulling in dask and dask-image (in fact I also ran into issues when packaging for WASM since there are issues with the dask recipes). I think it would make sense to support this, and there are a few options:

* Wait to see if [PEP 771](https://peps.python.org/pep-0771/) gets accepted
* Add a ``[dask]`` extras and make it opt-in – but this is not ideal since we allow blocked and parallel reprojection to be carried out without the user necessarily knowing we are doing this with dask, so if we wanted to do this option I think we'd want to implement a dask-less fallback for multi-threading (which might not be *too* hard)
* Split the package into reproject-core and reproject where the latter is a metapackage that pulls in reproject-core plus dask etc. - could do this easily infrastructure-wise with [rind](https://rind.readthedocs.io/en/stable/)

Review the three proposed approaches in this issue, including PEP 771 and rind, before choosing an implementation direction. The work is done when reproject offers a documented minimal installation path without dask and dask-image while retaining the needed reprojection behavior.

Written by the indexing model from the issue text.