"Backend" term is overloaded

[tomwhite]

"Backend" refers to:

1. the backend array API implementation (e.g. NumPy, JAX, CuPy), and
2. the backend Zarr store (e.g. Zarr, Tensorstore).

This can be confusing as we have open_backend_array for opening a Zarr store, and numpy_array_to_backend_array for converting from a storage array to a backend array API.

I propose that we remove the word "backend" from 2. and just call it a "store" or "storage". So open_backend_array would become open_storage_array, and numpy_array_to_backend_array would become storage_array_to_backend_array (I suggested the latter in #750).