Allow non-svelte files in layouts folder

[jimafisk]

Sometimes it would be helpful to include non .svelte files, like an SVG, in your layouts folder instead of putting them in your assets folder. Or maybe you want to include a simple .js helper, and while you could export this as a module in a Svelte file, there's no need to add processing time to the Svelte compiler. It would be nice if Plenti would simply copy these files over to the appropriate location in the public build folder. My only hesitation is when you offer more options, it invites an opportunity for conventions to become fuzzy.

[jimafisk]

I can't remember exactly when we introduced this, but you should be able to include plain .js files in your layouts folder and the build will automatically copy them over so you can reference them in a svelte template. If you want to manipulate SVGs it probably makes sense to embed them in a svelte template or sub-component. For static files like PNGs, JPGs, etc, it makes the most sense to just put those in your media/ folder.