[Powertoys Run] Add Base64 Decoding

### Description of the new feature / enhancement

Update PowerToys Run - Value Generator plugin to support Base64 decoding.
This is a subrequest for #19607

### Scenario when this would be used?

I often need to convert between base64 and plain text. The decoding would complement the Value generator's base64 encoding feature.

### Supporting information

_No response_