User defined theme file banner in CLI config

[mimarz]

Requested feature from Slack thread: https://designsystemet.slack.com/archives/C05RK9VEGJ0/p1745478413017869

A user might want to set the file header to describe more metadata used for generating a the theme file.

Add support for users passing in their own file header string.

Needs to be support in all three use types, from cli, config and js module.

[mrosvik]

Related: #4567

[eirikbacker]

This could be quite easy if we do #5104

[mimarz]

Closing this for now as we have not gotten any additional feature requests for this and we are looking at changing this behaviour.