Support for IDEs and platforms

I've added some nice-to-have features (which is what templates are all about!):

• Asking which platforms should be supported
• IDE-specific dependencies
• Platform-specific tasks for opening IDEs (currently just MacOS)
• VS Code workspace settings (e.g. path to pixi R installation)
• A basic templated README.md

R template for pixi

This is the first release of this pixi R template. It is currently in alpha stage. More info to come!