chore: refactor migration composition logic

[paoloricciuti]

Svelte 5 rewrite

I was working on the migration for svelte:component but the code to compose together the instance was getting a bit messy because we were trying to add stuff all in the same place if there was no script or one by one if there was one. I changed this a bit by using an insertion_point variable that we can set to then insert stuff sequentially. It seems to work fine except some minor changes in whitespace but i would love an extra look from @dummdidumm

I'm using this already locally to add the migration for svelte component.

P.s. i've also noticed another bug that i had to fix locally which is that we never visit the expression of svelte:component even in normal analysis. This means that if the expression is using $$props we don't have uses_props correctly initialized. Should i push this fix too?

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