Function isValidPostalCode

isValidPostalCode(code: unknown): boolean
Validates whether the given code is a valid 5-digit Sri Lankan postal code.
Parameters
- code: unknown
  Postal code to check.
Returns boolean
true if the code is five digits with a known district prefix. Exact post-office assignment is not checked.
Example
```
isValidPostalCode('00100'); // true
isValidPostalCode('abc20000'); // false
```
- Defined in postal-code/validator.ts:78