Document + support switching between native mode and transpiled mode

The only documentation for how to turn on native mode for Skip is to skip init --native-app or --native-model.

I've found it really useful even when ultimately targeting transpiled mode to do work in native mode, get my code working that way, and then transpile it. This ensures that my initial issues will all be in limitations with SkipUI with no transpilation issues, and then, when I go to transpile, I'll know that my code can work in SkipUI, and that the only issue is transpilation.

Today, I do that by copying and pasting source code between a native project and a transpiled project; that's a hassle. I wish there were a config setting I could use for this, instead, making switching between modes as easy as updating a config setting.

Or, if that's not feasible, I wish I could run a skip command to switch between modes without creating a fresh project with init.

Start with the existing documentation for skip init --native-app and --native-model, then determine how switching between native and transpiled modes should work without copying source between projects. Define whether a configuration setting or a skip command is the intended workflow, and verify that the chosen approach supports both modes.