feat: support for CH VATs

[Mollemoll]

Why this PR?

Looking to extend Valvat with CH VAT numbers.

What's changed?

• CH was used as a previous test case for VAT outside EU. This was changed to NO instead.
• Added CH as a supported state and added syntax validation
• Added CH Lookup through BFS. Default configuration is to always return false on CH lookup unless it's performed with the ch: true flag.
• Added checksum support for CH (mod 11 checksum)

Background

CH VAT Syntax

CH VATs can take different formatting variations. They start with CHE, followed by 9 digits and an optional suffix describing the regional part of Switzerland (MWST, IVA or TVA).

Format examples:

CHE123456789
CHE123456789 MWST
CHE-123.456.789
CHE-123.456.789 MWST

• Stripe
• Alavara

BFS Lookup:

• Does not accept CHE123456789MWST (without a whitespace before MWST)

Errors

According to Google Translate and the [Spécifications Webservice Interface 5.0 PDF](Spécifications Webservice
Interface 5.0) the ValidateVatNumber operation can only return two errors:

• Data_validation_failed
• Request_limit_exceeded

Sources

Lookup:

• BFS UIDs
• BFS SOAP API

Checksum details (weight and mod):

• Details on checksum
• PDF with checksum weights

[Mollemoll]

Hi @yolk,

Any idea when you'll have time to have a look? Looking forward to your feedback.

/Jonas