top-level API

Now that we have v2 and v3 support, we have the opportunity to define a top-level API that is agnostic over the v2 / v3 difference. That means we can have a `from_zarr(x)` function that does something like this:

```python
def from_zarr(x):
    if is_zarr_v2(x):
        return v2.from_zarr(x):
    if is_zarr_v3(x):
        return v3.from_zarr(x)
```