Provide virtual input file system to .build()

## Description

Allow an initial partial filesystem seed to be provided as an input to a `.build()` operation.

## Use-case

In order to increase the common capabilities between the WASM-based and binary-based implementations, it would be helpful to have some measure of control over the filesystem. I thought that one potential avenue for this that would avoid expensive and complicated coordination between `esbuild` and the controlling system might be to allow an initial FS snapshot to be seeded.

For my use-case, this would allow me to fully embrace `esbuild-wasm` and have platform and runtime environment portability.