More refinement of webfs impl

[jasnell]

Reworks the Webfs implementation to be more correct and pass more of the WPT's.

Simplifies things a bit by removing the sync fs api for now (we can add it back in later)

[github-actions]

The generated output of @cloudflare/workers-types matches the snapshot in types/generated-snapshot 🎉

[jasnell]

Btw, the biggest change here is that the WebFs classes no longer directly hold the kj::Rc<...> for the underlying fs node and instead will look it up each time an operation is called... which is what the spec actually expects. Per the spec, and per the tests, it is actually possible to acquire a handle for a node, have that node be deleted and recreated, and the handle would still be viable/usable. The prior implementation failed that expectation.

This also still isn't perfect, as there are a few other bits in here that need to be fixed, but I'm trying to go through these in smaller, more consumable/reviewable chunks rather than all at once.