feat(format): add Norwegian account number formatting

[geirarne]

Summary

Adds Norwegian bank account number formatting to @obosbbl/format as a reusable formatter API.

Changes

• Adds formatAccountNumber to the Norwegian formatter entrypoint.
• Exposes formatAccountNumber from the default entrypoint for locale: 'no'.
• Documents the new method in the package README.
• Adds formatter test coverage for raw, already formatted, dotted, and non-matching input.
• Adds a minor Changesets entry for @obosbbl/format.

Context

This is the upstream follow-up for code-obos/styrerommet#9609, where norwegian-utils is being replaced and account-number formatting was identified as generally useful formatting behavior that belongs in @obosbbl/format rather than only in the Styrerommet shared package.

Validation

• pnpm run lint:ci
• pnpm run test:ci
• pnpm run build