Twig extension test

## Changes
Adds `includeVat()`, `excludeVat()` and `loadVatRate()` to `VatService` interface.

Implements the `ezprice_value` twig extension. Given a price fieldId + versionInfo, it applies the rate depending on `isVatInclude`, and returns an enriched `Price\Value` that contains `price_included_vat` and `price_excluding_vat` properties.

## Testing
The twig extension is covered with a twig integration test that covers prices with and without vat included.