Add initial KubeJS setup with README, configuration files, and example scripts

- Created README.txt with directory structure and usage information.
- Added common.json, client.json, and web_server.json for configuration settings.
- Introduced example server and startup scripts (main.js) for demonstration.
- Included example textures for blocks and items in the assets directory.

kubejs/startup_scripts/main.js

@@ -0,0 +1,3 @@
+// Visit the wiki for more info - https://kubejs.com/
+console.info('Hello, World! (Loaded startup example script)')
+