Migration script

[Rich-Harris]

This adds a new package, svelte-migrate, which will automate the automate-able bits of #5748:

npx svelte-migrate routes

It doesn't attempt to boil the ocean — it just moves files to their new homes and annotates the code with manual TODOs, which will be documented in a GitHub discussion.

(For clarity, we're doing the migration script first so we can dogfood it on our own apps and test suites, but we haven't implemented the changes yet, so don't get ahead of yourselves and run this on your own apps just yet!)

Bunch of stuff that still needs to happen:

• fail gracefully when dealing with code that can't be parsed by TypeScript
• add the documentation
• inject errors into +page.server.js and +server.js telling people to update handler return values
• inject errors into +page.js and +layout.js telling people to update load return values
• inject error telling authors to replace multiple props with a single data prop
• deal with the fact that +error can't have load, somehow
• adjust imports in component code
• replace ./__types/whatever with ./$types

[changeset-bot]

⚠️ No Changeset found

Latest commit: 813c9ed

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[Rich-Harris]

This is a bit loosey-goosey (I didn't add tests, on the basis that this is somewhat throwaway, but I could be persuaded of their value) but probably far enough along that we can start trying it out on codebases to see how far we get.

[Rich-Harris]

alright, time to give this a whirl