Feature: Top level V3 API

[jhamman]

Marking as Work In Progress for now. Currently waiting on #1670 to merge.

Closes #1598 Fixes #1019

Open questions I could use feedback on:

1. I've combined zarr.creation and zarr.convenience into a common API module. This makes sense to me because they are all basically wrappers that help us construct Group/Array objects. What people think about this?
2. I've created parallel synchronous and asynchronous modules with basically identical APIs. All the synchronous functions simply call their asynchronous cousin. I like this much more than having two versions of each function (e.g. ones and async_ones). Thoughts?

More soon.

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/tutorial.rst
• [ ] Changes documented in docs/release.rst
• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)

cc @d-v-b, @aldenks