`variant_get_json`: Initial implementation

[sdf-jkl]

follow up to #32

variant_get_json(VariantArray, path)

Returns a nested raw JSON string from a Variant by path.

Arguments

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

Returns

• Json string (nullable)

Notes

This is a convenience wrapper equivalent to:
variant_get(variant, path, 'Json')

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

[sdf-jkl]

Existing src/variant_to_json.rs is very similar but does not use path to find the value.