`variant_get_bool`: Initial implementation

`variant_get_bool(VariantArray, path)`

Returns a Boolean value from a Variant by path.

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

## Returns
- Boolean (nullable)

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

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