feature request: support a virtual filesystem that is both readable and writeable.

> [Filesystem](https://github.com/bytecodealliance/WASI-Virt#filesystem): Mount a read-only filesystem, configure host filesystem preopen remappings or pass-through.

Similar to this use case https://github.com/bjorn3/browser_wasi_shim?tab=readme-ov-file#usage
The `stdin`, `stdout`, `stderr` are handled by the Xterm terminal library, while the files are stored as `Uint8Array` in a tree structure in-memory.

The virtual filesystem should allow both reads and write and be flexible enough to allow any backend (in-memory, Xterm, network requests, etc.)

Start by reading the WASI-Virt Filesystem reference and the browser_wasi_shim usage example linked in the issue. Then inspect the repository's existing WASI API implementations and determine how a read/write filesystem could support in-memory, Xterm, and network-backed storage. Done means the scope and backend interface are agreed and both reads and writes work through a virtual filesystem.

Written by the indexing model from the issue text.