docs: update requirements for generated types

[homostellaris]

Clarifies that Typescript is required as a dependency for generated types to be created. When the dependency is missing they are silently omitted. This requirement may not be obvious to those who are running a vanilla JavaScript project and have not used the scaffolding script (e.g. because they migrated from an earlier version of SvelteKit).

I came across this issue because I migrated from an alpha version and spent a while scratching my head before realising why the new generated types were not working for me.

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[benmccann]

Suggested change

	> For this to work, your own `tsconfig.json` or `jsconfig.json` should extend from the generated `.svelte-kit/tsconfig.json` (where `.svelte-kit` is your [`outDir`](configuration#outdir)) and typescript should be installed as a dependency:
	> For this to work, your own `tsconfig.json` or `jsconfig.json` should extend from the generated `.svelte-kit/tsconfig.json` (where `.svelte-kit` is your [`outDir`](configuration#outdir)) and TypeScript should be installed as a dependency:

[homostellaris]

Done 👍

[homostellaris]

Is this good to go now?