Initial restructuring of kvvfs working towards generic support of JS's Strorage inter...

Initial restructuring of kvvfs working towards generic support of JS's Strorage interface as storage rather than hard-coding it to just window.localStorage and window.sessionStorage. The eventual goal of that is to have a storage solution which works without the POSIX I/O APIs, getting us one step closer to a wasi-sdk build. It would be transient, like Emscripten's virtual filesystem, and could hypothetically replace our dependency on that particular feature.

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