docs: add missing type information

[eltigerchino]

This PR adds missing type information to several code blocks with the benefit being that users get type information on hover and we get validation when building the app. It also inlines the type information for the service worker example so that it's easier to copy and paste for new projects (it provides a js/ts toggle too where there previously was none https://svelte.dev/docs/kit/service-workers#Inside-the-service-worker).

EDIT: this PR should be merged before sveltejs/svelte.dev#1398 which ensures TypeScript info is correct in the docs

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-13928-svelte.vercel.app/