Implement #5748

[Rich-Harris]

Edited: this now incorporates #5782 and #5810.

This PR implements #5748. The migration guide is available at #5774.

• use the app's installed TypeScript rather than including it in SvelteKit (i.e. don't generate types otherwise)
• create types for parent
• make write_types work incrementally
• update the docs
  • document POST, PATCH, PUT and DELETE in +page.server.js
  • get search working for +page etc
  • 'The following types can be imported from @sveltejs/kit' is wrong in the modules section
• get all the tests passing
• ensure all the migration guides are correct
• investigate why client-side navigation fails on the preview deployment
• rename GET to load in +page|layout.server.js (POST etc can stay, pending resolution of Semantic form actions, and easier progressive enhancement #5875)
• remove session, per Removing `session` #5883
• implement invalidate() (i.e. re-run all load functions, if no argument is specified)
• update types inside migrated files (+page.server.js should have PageServerLoad etc)
• error if project has not been migrated
• mother of all changesets (including for svelte-migrate and create-svelte etc)

[changeset-bot]

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Latest commit: 99690ae

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