Function isValidBankCode

isValidBankCode(code: unknown): boolean
Validates whether the given code is a valid 4-digit CBSL bank code.
Parameters
- code: unknown
  Four-digit CBSL bank code, as text or a number.
Returns boolean
true if the code belongs to a bank in the CBSL table.
Example
```
isValidBankCode('7010'); // true
isValidBankCode('0000'); // false
```
- Defined in bank/validator.ts:18