We don't expect any return value from the underlying `fsync()` so fix
the function signature accordingly.
Fixes the following obscure error:
```
TypeError: Cannot convert undefined to a BigInt
at wasm://wasm/00942492:wasm-function[1501]:0x1c4dc1
at wasm://wasm/00942492:wasm-function[694]:0x189586
at wasm://wasm/00942492:wasm-function[50]:0x143d7
at wasm://wasm/00942492:wasm-function[60]:0x3f91a
at new Database (/Users/penberg/src/penberg/limbo/bindings/wasm/pkg/limbo_wasm.js:162:26)
at file:///Users/penberg/src/penberg/limbo/bindings/wasm/perf/perf-limbo.js:5:12
at ModuleJob.run (node:internal/modules/esm/module_job:195:25)
at async ModuleLoader.import (node:internal/modules/esm/loader:336:24)
at async loadESM (node:internal/process/esm_loader:34:7)
at async handleMainPromise (node:internal/modules/run_main:106:12)
```
This adds a basic VFS interface to use Node filesystem API from Rust
code. Not a lot, but parses the SQLite database file header and
completes database open without errors.
