Fixes a variety of issues related to string data types.

• Brings the VariableLengthUTF8 data type Zarr V3 identifier in alignment with Zarr Python 3.0.8
• Disallows creation of 0-length fixed-length data types
• Adds a regression test for the VariableLengthUTF8 data type that checks against version 3.0.8
• Allows users to request the VariableLengthUTF8 data type with str, "str", or "string".