Support fd_sync for stdout/stderr writers via optional interface

[robherley]

As far as I know, when an arbitrary io.Writer is passed for the module's stdout/stderr, it's not possible to hook into fd_sync when it is called on the respective file descriptor.

I've extended writerFile struct to attempt to call a Sync method on the inner writer interface, if it passes the type assertion.

My use case is as follows:

1. For the module's WithStdout, I'm passing in a wrapper http.ResponseWriter.
2. When the guest calls os.Stdout.Sync() I want to invoke http.Flusher.Flush() on the guest.

As long as the writer decides to implement this simple interface:

type syncer interface {
	Sync() error
}

The host can hook into whenever the client flushes the fd.

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Also, thanks for this amazing project, I've been meaning to dive into wazero ever since I saw Takeshi's talk at GopherCon '22 in person 😄