Added Watermarker extension json

[Xavia1991]

Info

A lightweight extension for the extra tab.
Adds option to add a scalable watermark with adjustable alpha at corners or center of an image when upscaling.
Adds setting for default watermark image.
quick and easy watermarking without damaging the metadata.
https://github.com/Xavia1991/Watermarker

Checklist:

• I have read the Readme.md
• The description is written in English.
• The index.json and extension_template.json have not been modified.
• The entry is placed in the extensions directory with the .json file extension.