Run preprocessors on svelte components when using svelte-kit package

[AsyncBanana]

Is your feature request related to a problem? Please describe.
Running svelte-kit package does not run preprocessors, which makes things like PostCSS unable to work.

Describe the solution you'd like
svelte-kit package should run the preprocessors specified in svelte.config.js in .svelte files like svelte-kit build does.

Describe alternatives you've considered
Manually copying over the files, or coding a script to do it.

How important is this feature to you?
It is very important because I like using svelte-kit package much more than plain Svelte setups for libraries. I like it more because it removes the need to set up build tools and includes a way to easily test and build a documentation website. Unfortunately, I am unable to use it efficiently because my CSS can't compile automatically.