chore: support function for compilerOptions

[dummdidumm]

For svelte2tsx we gotta make a compromise - since everything has to be sync, we can only pass the unpreprocessed file. Should be fine in practise because a preprocessor influencing the code in such a way that it in turn influences the compiler options is probably vanishingly rare.

https://github.com/sveltejs/svelte/issues/12415