writing files as unpacked

there currently is support for unpacked files in the reader, but not in the writer.

not sure what should it look like in terms of the api. in electron_tasje, i'm currently writing "unpacked" files both in the asar and separately, which works enough but makes them take double the space

Look at the existing reader support for unpacked files in the codebase to understand the format. Then examine the writer module to see how files are currently packed. The API design needs to be decided: check similar tools or the Electron asar spec. Implement writing unpacked files without duplication, and test with sample archives.

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