Request for appendFile method

Hi,

I try to optimize the time to load big files. For the moment, I follow a 2 step approach :+1:

- First I read the input file in a memory buffer (UINT8Array)
- Then I write the contents of the buffer to FFMPEG fs using ffmpeg.FS.writeFile method.

However I think it would be faster reading the source file into chunks with File Stream Reader and then writes directly these chunks in append mode to the FS object.

Unfortunately ffmpeg.FS does not expose the appendFile method, or even better it would be great to have a File Stream Writer.

Thanks,
Stéphane