Add xielu activation function

[Goekdeniz-Guelmez]

Proposed changes

from Sweden's Aperus model, this is still too niche, I do have a gut feeling that this would get more traction, but not now, we can merge this when its more well known, until then its in draft mode.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)