[feature request] Modernize file_package.py: chunking; promise-based loading and file creation; caching/decompression -> library-js; JSON-based index

Grouping earlier issues:
- [ ] https://github.com/emscripten-core/emscripten/issues/14362
- [ ] https://github.com/emscripten-core/emscripten/issues/12347
- [ ] https://github.com/emscripten-core/emscripten/issues/12342
- [ ] https://github.com/emscripten-core/emscripten/issues/18445

Currently it uses timers / dependenciesFullfilled callback which are not controlled by the user.

Maybe good first steps for an implementor:
- move out helper methods (mainly IDBFS caching / LZ4 decompression) produced by file_packager.py into a JS file that can be included with the main JS library
- file_packager.py should also export a JSON containing file-list and data offsets