feat: add app-state migration

[dummdidumm]

A simple, string-based migration for sveltejs/kit#13140 (tested on that repo, zero false-positives, less than five false negatives)

We can switch to using the Svelte parser if people prefer that to get a bit better with regards to only actually transforming identifiers, but to me this seems to work fine.

[changeset-bot]

🦋 Changeset detected

Latest commit: 25a948b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte-migrate	Minor

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commit: 25a948b

[Rich-Harris]

preview: https://svelte-dev-git-preview-cli-354-svelte.vercel.app/

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[manuel3108]

Merged main here after merging #349 to have more sensible ordering of the migrations in the readme and website. Added the new migration to docs. Feel free to change the wording

[Rich-Harris]

Should we make this an app-state migration and include navigating and updated?

[dummdidumm]

This already contains migrations for navigating (though the one with current on it) and updated - page-state is a misnomer, you're right that we should rename it to app-state

[benmccann]

Heads up that this only found 9 out of 22 occurrences of $app/stores when I ran it on a real project. I filed an issue here: #368