fix: ISK currency formatting

[Thithip]

Summary

When processing a payment in ISK (Iceland) currency, the amount displayed on the Stripe Widget is wrong by adding two extra zeros.
This PR add a special case for the ISK currency.

Motivation

Resolves #3986

Testing

Add a test case in the test file NSString+StripeTest

Changelog

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[davidme-stripe]

Thanks for fixing this! Sorry for the delay, we'll get this merged.

[Thithip]

Thank you David :)