Allow optionally compressed VFS image data

[georgestagg]

Uses pako for fast decompression of VFS images before mounting.

In principle static file web servers can serve content gzip compressed, but in practice it turns out services like GitHub Pages only compress certain types of object, likely based on MIME type.

From experimentation, it looks like VFS images with names ending in .data are not compressed over the wire, and so pre-compression with gzip can be useful when deploying webR to such services.

Using pako should be faster than R's built in .tar.gz decompressor, and is not a heavy prerequisite.

Note: Using Emscripten's --lz4 in file_packager is not a great fit here, because we do not directly execute the .js output from that tool. Instead we manually mount the image using MEMFS.mount(). Unfortunately, with that workflow the .js.metadata file does not actually contain all the required metadata to decompress the VFS contents.

Note: Once merged and included in a release, {rwasm} can be tweaked to include compressed R packages. Then, once R 4.5.0 is released we can turn off the uncompressed output.