fix: accept Nigeria's 11-character postcodes

[Adeniyikayodee]

Fixes Or Enhances

NIPOST launched a new postcode for Nigeria in October 2026. It has 11 characters in five segments, for example EK-01-A03-FK-01: two letters, two digits, three letters or digits, two letters, and two digits. The two numeric segments run from 01 to 99.

postcode_iso3166_alpha2=NG accepted six digits only, so every new code was rejected. The rule now accepts both. The new code may be written with hyphens, with spaces, or with no separator, in upper case like the other rules in the file.

Format reference: https://docs.postcode.gov.ng/concepts/postcode-format

go test ./... and go vet ./... pass on Go 1.27.1.

This change was prepared with AI assistance (Claude) and reviewed by me.

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[Adeniyikayodee]

@nodivbyzero, I would be grateful if you could look at this whenever your time allows. Nigeria launched a new national postcode on 1 October, and the NG rule still accepts only the six-digit code it replaces, so every new code fails postcode_iso3166_alpha2. The format and its source are in the description, and the tests cover both codes. I am glad to change anything that would make this easier to merge. Thank you for the work you put into this library, and please take it at your own pace.

[Adeniyikayodee]

Thank you, @nodivbyzero, for the review and the merge.