Use browserfs/zenfs to overlay one filesystem on top of another

[adamziel]

What is this PR doing?

Experiment related to #1379

Uses the zenfs library to create a layered copy-on-write filesystem.

Upsides

• Enables "mounting" single files by overlaying MemoryFS on top of an underlying NodeFs mounts
• OverlayFS seems to just work and even implements complex bits like marking parts of the writable tree as deleted even if the deleted paths didn't actually exist in it
• Not having to go through the complex process of implementing a custom Emscripten Overlay Filesystem

Downsides

• Introduces additional complexity
• Zenfs implements Filesystem mechanics from scratch and the semantics might differ from the Emscripten backend in a nuanced way. Also, it's another layer of abstraction which might slow things down. This would require rigorous testing before merging.

Remaining work

• Use Node.js fs module for zenfs writable FS.
• Figure out why reading files from Zenfs through Emscripten mount yields a buffer filled with zeros.

Testing instructions

Run this to trigger the code path that mounts ZenFS:

bun packages/playground/cli/src/cli.ts server --skipWordPressSetup

It doesn't do much interesting stuff yet, but at least it mounts the Overlay FS

[adamziel]

I'll close this PR – it was a useful exploration we can reuse in the future, but I don't intend on pursuing it now.