[v2] Add support for default chunk shape and auto-detection of dimension separator

[brokkoli71]

In JZarr, there are some convenience features that are currently missing in zarr-java. Implementing them would improve usability and compatibility.

1. Default chunk shape selection

   • When chunkshape is not specified during array creation, JZarr automatically selects chunks of about size 512 (with some logic to make it fit).
   • Reference implementation: ArrayParams::build.

2. Auto-detection of dimension separator

   • If an array is opened without a specified dimension_separator, JZarr can infer the correct separator from the filesystem layout.
   • Reference implementation: ZarrArray::findNestedChunks.