Add Czech and Slovak IČO validation

[DMZ22]

This implements #450: validation for the IČO, the organisation identifier used in the Czech Republic and Slovakia.

Format

• Name: IČO — Identifikační číslo osoby (CZ) / Identifikačné číslo organizácie (SK), also written IČ.
• Structure: 8 digits, the last being a weighted modulo-11 check digit (weights 8…2 over the first 7 digits, check digit (11 - sum) % 11, mapped so a remainder of 0 → 1).
• Identifies a legal entity or sole trader; listed in the Czech ARES register and the Slovak ORSR / RPO registers.

Relation to DIČ (raised in the issue)

For a legal entity the IČO is exactly the digits of the Czech VAT number (DIČ) without the CZ prefix, so cz/ico uses the same check-digit computation that cz/dic already applies to its 8-digit legal-entity form. The Slovak IČO is identical to the Czech one (until 1993 they were a single country), so sk/ico delegates to cz/ico, mirroring the existing sk/rc → cz/rc pattern.

Sources

• https://cs.wikipedia.org/wiki/Identifikační_číslo_osoby
• Official register / validation service: https://ares.gov.cz/ (CZ), https://www.orsr.sk/ (SK)

Tests

• Module docstrings cover valid input, invalid check digit, wrong length and non-digit input.
• tests/test_cz_ico.doctest and tests/test_sk_ico.doctest each bulk-validate 21 real numbers taken from public company records (e.g. Škoda Auto 00177041, ČEZ 45274649; Slovnaft 31322832, Tatra banka 00686930).
• All doctests pass under NORMALIZE_WHITESPACE/IGNORE_EXCEPTION_DETAIL; every line of the new modules is exercised (100% coverage).

Happy to add a format() grouping or an IČO → DIČ helper if you'd find either useful.