Simplify Vite config

[Rich-Harris]

Another stepping stone on the way to #7967 (which at this point is less important than the codebase improvements it's driving).

Right now the vite directory is a bit of a mess. The code is full of indirection, and the logic is split up in ways that are quite hard to follow. This simplifies everything.

The vite-plugin-sveltekit-compile plugin should be renamed vite-plugin-sveltekit-serve, though that would break the Storybook integration, which removes that plugin from the array.

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Latest commit: f1ac8468b17692700b9f9feb4156edfef32c718f

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[Rich-Harris]

Reverted it to a single plugin and moved the define stuff around, even though the code is more confusing now.

I really dislike the concessions we're making here. Something is deeply wrong with how these tools interact with each other. I don't know what the correct solution is, but mutating an array of plugins to remove one that has a specific name is definitely not it.