change extension to '.user.js' for easy install

[razetime]

When a userscript is named .user.js, opening the raw version of the userscript will automatically open a Tampermonkey page with an option to install the extension. For an example you can see https://github.com/razetime/userscripts/raw/main/staxupload.user.js (click 'cancel' to avoid installing)

[darkmain]

And a pull requests already exists: #47 ;)