Function isBase64

isBase64(s): boolean
check whether a string is a base64 encoded string or not
Parameters
- s: string
  string to be checked
Returns boolean
true if the string is a Base64 encoded string
See
isBase64URL

Example
```
isBase64("YWE=") -> true
isBase64("YW_=") -> false
isBase64("YWE") -> false // length shall be multiples of 4
```
- Defined in checker.mts:28