feat(CountryCode): add support for Kosovo country code

[Ignigena]

Description

This adds XK as a valid country code to support Kosovo (see #2028)

This is the hard-coded/brute-force method -- it may be worth keeping the existing regex constrained to official assignments and just relax the restrictions to allow for the user-defined set in the standard: AA, QM to QZ, XA to XZ, and ZZ.

Please feel free to decline if the latter solution is preferred.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have followed the CONTRIBUTING doc and the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests and linter rules pass locally with my changes
• Any dependent changes have been merged and published in downstream modules