Basic filesystems support for Wasm: reuse JS implementation for wasmJs target

[fzhinkin]

This PR consists of the first patch bringing basic filesystem support to Wasm targets:

• splits the single Wasm source set into wasmJs and wasmWasi;
• creates a separate source set with nodeJs-filesystem implementation shared between js and wasmJs targets.

[fzhinkin]

the first patch

The second part will be about wasmWasi :)

[lppedd]

Nice!

@goncalossilva you might want to use this instead of your custom implementation for JS/WASM in kotlinx-resources