Add version to composer.json

[timotheegoguely]

Kirby plugin best practices:

Always add a composer.json file with basic information about your plugin, including the current version number, so that Kirby can use this information in the Panel's System view, and users can easily get information about your plugin with Kirby's plugin methods.

https://getkirby.com/docs/guide/plugins/best-practices#add-composer-json