chore: undeprecate config.kit.files.* options

[teemingc]

As discussed during the maintainer's meeting, this PR undeprecates the kit.files options. It also reverts the removal of mentions about these options in various parts of the docs done in #15343

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-15482-svelte.vercel.app/

[dummdidumm]

I think we should keep it hidden from the docs mostly, so that people don't stumble across it too much.
We also said that we wanted to keep it deprecated in the types along with an explanation that this is a "soft deprecation" in the sense of it's still supported but no longer recommended (use monorepos for most use cases instead)

Most importantly we need to revert the deprecation from the options parsing

[teemingc]

Thanks! I made the changes but I'm unsure about the deprecation wording in the types