add tax category code for free export ("G") and add it to "with exemp…

[timkarliczek]

When a Product is used with setTaxCategoryCode('G') ("Free export item, tax not charged") setTaxExemptionReason("Steuer nicht erhoben aufgrund von Export außerhalb der EU") i still get the following validation error:

[BR-G-10]-A VAT Breakdown (BG-23) with the VAT Category code (BT-118) "Export outside the EU" shall have a VAT exemption reason code (BT-121), meaning "Export outside the EU" or the VAT exemption reason text (BT-120) "Export outside the EU" (or the equivalent standard text in another language).

This happens because ZUGFeRD2PullProvider sets <ram:ExemptionReason> only if the categoryCode is set inside CATEGORY_CODES_WITH_EXEMPTION_REASON. So i added "G" there.

Source of the screenshot: Factur-X 1.0.07_Technical_Appendix_Profile_EN16931.pdf downloaded from https://www.ferd-net.de/standards/zugferd-version-archive/zugferd-2.3.2.html