`variant_get_array`: Initial implementation

follow up to #32 

`variant_get_array(VariantArray, path)`

Returns an Array value from a Variant by path.

## Arguments

- VariantArray -- a list of variant values
- path -- the variant path string

## Returns

- Array (nullable)

## Notes
This is a convenience wrapper equivalent to:
`variant_get(variant, path, 'Array')`

Behavior (null/error/casting) should match `variant_get` with a type hint.