Mixpanel token should be saved on JS front

added a MR for this issue.

I agree this is a good idea, looking good so far, just one comment to address :)

Looking good, thanks. Doesn't quite adhere to coding standards, if I turned on phpcs I reckon it would complain but at the moment I don't mind with this module.

I'd like feedback from someone else manually testing that this works or even better an automated test.

- Added JS test for frontend project token
- Needed to include the core/drupalSettings library in case it is not included anywhere else
- Tidied up the module file a bit to better meet coding standards