feat: add bundleStrategy option

[paoloricciuti]

This adapts the code from @benmccann branch https://github.com/sveltejs/kit/tree/iife to the new shape of the repo. I also had to make some other changes namely:

1. instead of using the iife output (which doesn't export anything and put everything in a global variable) i used manualChunks with a single chunk
2. since we have everything in a single file i "namespaced" app by export * as app
3. on the client when resolving the bundle i've assigned kit.app as a fallback for app

I'm not super-familiar with the test suite so if you can suggest how and were i can test this (i guess it should be a new app?) i would appreciate (also not really sure if this has any repercussions).

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13173-svelte.vercel.app/

this is an automated message

[Rich-Harris]

Going to change this to bundleStrategy: 'split' | 'single' rather than codeSplit: boolean, so that we can accommodate more options in future

[paoloricciuti]

Going to change this to bundleStrategy: 'split' | 'single' rather than codeSplit: boolean, so that we can accommodate more options in future

If you want I can do it myself

[Rich-Harris]

it's all good, I'm in the branch — I'm laying the groundwork for a separate feature that will be built on top of this, so there might be additional tweaks required