feat: Add base64 encode and decode variables (#3186)

[TheStaticMage]

Description of the Change

This PR adds two new base64 related variables: $base64Encode and $base64Decode, and two new variables that alias those: $btoa and $atob respectively.

Note: The implementation here should work properly with emojis or other multi-byte Unicode characters as well (tested this).

Note: The original issue (which I wrote) had these as base64encode and base64decode, but looking at the other variables in that directory, they seem to use camelCase, so I did too.

Applicable Issues

Fixes https://github.com/crowbartools/Firebot/issues/3186

Testing

Tested this in a chat feed alert. See screenshot.

Screenshots

This is in the log:

[2025-06-19 06:32:11.0701] - error: Failed to decode base64 string: !