feat: v2 migration helpers

[dummdidumm]

• svelte-migrate stuff
• migration docs

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[benmccann]

I think we were a little inconsistent in the language of some of the changesets. Maybe we can align them as part of this. Especially .changeset/gentle-guests-clean.md where I said "drop support for Svelte 3" I think maybe we could word in a more upbeat way like "require Svelte 4 to ease the path for Svelte 5 migration" or something.

[benmccann]

I think it'd be nice for this to appear before "Migrating from Sapper" in the sidebar since it will be relevant to more users. And maybe we can call it "Migrating to SvelteKit v2" rather than "Migration to SvelteKit v2" for consistency, so that both entries begin with "Migrating"

[benmccann]

can we also make sure typescript is on v5 and that moduleResolution is bundler for applications and NodeNext or Node16 for libraries?

[dummdidumm]

AFAIK NodeNext is already used for libs for a while already, but we can set bundler if we see nothing is set, or better yet remove the setting entirely.
Typescript should already be ^5.0 through the Svelte 4 upgrade

[benmccann]

Okay. One thing to note is that the v2 branch sets it to NodeNext when running package, which isn't done on master, but it's kind of weird because it might not match what's in the tsconfig.json. Maybe we should have package check if that's the value contained in the tsconfig.json?

[dummdidumm]

Merging now and doing the other changes in their respective PRs sounds good.
Things I have in mind for migration that are part of already merged PRs, which could maybe added here before merging:

• add path to cookies methods where needed/possible
• more tsconfig tweaks like removing the moduleresolution and module if it's outdated settings (node)
• remove dangerZone from svelte.config.js

[Rich-Harris]

Am trying to insert // @migration task: Specify path comments and I'm just totally lost. Seems like a simple enough thing to do but I can't see any APIs for prepending text, only replacing (which fucks up indentation). Frankly I suspect we'd have a much better time if we ditched ts-morph in favour of using the same traversal techniques we use in svelte (i.e. using acorn with acorn-typescript to generate an estree AST which is much nicer to deal with than TypeScript insanity).

I've wasted enough time fighting with it and there are more important things to do, so I'm just doing to check in my work so far and merge this PR. Those other changes can be follow-ups.