add zarr tests

[MMesch]

Basic zarr tests as in this PR are currrently failing. I'm opening to iterate on it and get it running. Currently this is based on the latests zarr.

Would be itnerest to hear feedback from @DerThorsten and @davidbrochart .

[davidbrochart]

Zarr uses threads to do non-blocking disk I/O, so that can't work in WASM. I don't know if there is a way to configure it so that it doesn't use threads.
I ported watchfiles to WASM in anychange, it would be a similar change.

[MMesch]

What does work is:

dependencies:
  - zarr < 3
  - numcodecs < 0.16

Can we expect any problems using zarr < 3?

[davidbrochart]

It looks like in v3 they use a thread pool executor internally, which was not the case in v2.
I don't know if we can still use v2. At least v3 mentions pyodide compatibility: https://github.com/zarr-developers/zarr-python/releases/tag/v3.0.0.

[MMesch]

zarr > 3 issue on pyodide pyodide/pyodide-recipes#227

[MMesch]

recipe is pinned to <v3 https://github.com/pyodide/pyodide-recipes/blob/main/packages/zarr/meta.yaml

@DerThorsten we should maybe do the same?