Add runtime i18n hooks for validation messages

[aleksandr-kotlyar]

Summary

Adds runtime i18n support for Voluptuous built-in validation messages.

This PR introduces gettext-compatible hooks so applications can translate Voluptuous error messages without Voluptuous shipping production translation catalogs.

Main changes:

• Add configure_i18n() for loading gettext catalogs from an application-provided localedir
• Add set_gettext() for injecting a custom translation callable
• Add gettext_scope() for temporary context-local translation overrides
• Route built-in user-facing validation messages and the Invalid.__str__ message/error-type format through the i18n hook
• Keep the error path suffix, for example @ data[...], untranslated so existing tooling can continue to parse it
• Document the i18n API in the README
• Add test-only locale fixtures and packaging checks to ensure fixture catalogs are not treated as production catalogs

Voluptuous intentionally does not ship production translation catalogs in this PR. Applications are expected to provide their own gettext catalogs.

Also includes a few small validation correctness fixes covered by tests while touching error-message paths:

• enum scalar schemas
• set element transforms
• non-numeric Number input
• Schema.extend(Schema(...))
• Invalid.__repr__

Resolves #73.