perf: better api for std::fs::write

[Boshen]

The current api is not memory friendly for std::fs::write:

    std::fs::write(file_path, asset.get_source().buffer()).map_err(rspack_error::Error::from)?;

The buffer currently concats all the source code into a single String and then write to disk, this creates a huge memory pressure because we have to constantly reallocate for this String,
It would be better to expose a better api to work with BufWriter, where we can balance memory + fs::write syscall.

[Boshen]

Evidence:

[Boshen]

pub fn to_writer<W: Write>(&self, w: W) -> Result<()> should be good.

[Boshen]

After the change in #42