Function normalizePostalCode

normalizePostalCode(input: unknown): string | null
Normalizes a postal code into a 5-digit string (e.g. 100 -> '00100').
Parameters
- input: unknown
  Postal code as text or an integer. Shorter codes are padded to five digits.
Returns string | null
The five-digit code, or null if the input is not one to five digits.
Example
```
normalizePostalCode(100); // '00100'
normalizePostalCode('20000'); // '20000'
normalizePostalCode('2000A'); // null
```
- Defined in postal-code/validator.ts:19