Assign dimension names to meta_doc attributes

[TheJeran]

In Zarr v3, the dimension_names attribute is usually not in the attributes subgroup. So there's no way to get the dimension information of a given variable with v3.

This very simple fix just puts it in the attributes object so it's referenced with

get attrs() {
    return this.#metadata.attributes;
}

In the Array class.

[changeset-bot]

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[lazarusA]

note that this field is specified here:

zarrita.js/packages/zarrita/src/metadata.ts

Line 90 in 5fd73b6

dimension_names?: string[];