deprecate `to_dict`

[d-v-b]

Our Metadata base class defines to_dict and from_dict methods that are annotated as returning / taking dict[str, JSON]. This is problematic for two reasons:

1. Codecs and data types can appear in Zarr V3 metadata as strings. So from_dict needs to accept strings, which makes the name of the method confusing.
2. Zarr metadata is structured, but dict[str, JSON] cannot capture that structure. Typeddicts are a natural fit for modelling the exact structure of Zarr metadata documents, but typeddicts are not assignable to dict[str, JSON], but rather Mapping[str, object]. So the dict[str, JSON] type is a blocker for using typeddicts with to_dict/ from_dict (or whatever we call this method).

To get around these problems, I recommend:

• defining new methods to_json, from_json . These methods take / return a type assignable to JSON = str | None | int | float | tuple[JSON, ...] | Mapping[str, object]. I think that's the best JSON type we can do until the assignment rules for typeddicts change.
• deprecating to_dict and from_dict, and replacing their usage with to_json and from_json.

I already did this with the data types, and it's worked fine. One complication for the data types is that we have 2 JSON serializations to handle: v2 and v3 JSON flavors. I addressed that with a zarr_format kwarg the dtype to_json / from_json methods. But for something like a codec, we can actually inspect the type of the input to check if it's a v2 or v3 codec, so we don't need the zarr_format kwarg.