Move kotlinx.io.files package to a separate module

[fzhinkin]

No description provided.

[whyoleg]

JIC: there is one usage of kotlinx.io.files in ktor production code (not in tests) blindly (not thinking about consequences) introduced by myself in ktorio/ktor#4192
It was needed to not reimplement this from scratch (or copy from kotlinx-io) for js/wasm-js targets - previously it was just expect/actual
Probably it's better to drop this usage in ktor to avoid breaking changes? cc @e5l

[fzhinkin]

In short term, the change should not break Ktor as long as a new artifact is added to dependencies.