Use WASM commands

[ianthomas23]

This PR uses WebAssembly commands rather than TypeScript ones. It follows on from the experiment in jupyterlite/terminal#15 by moving the WASM modules from there to here. There are 2 WASM modules, fs.js contains the Emscripten filesystem used by the commands and a single instance exists for the duration of the shell, and coreutils.js contains many commands. The latter is reused for each command run and the filesystem from fs.js is patched in as a PROXYFS so that all commands see the same filesystem. There is no use of a JupyterLite DriveFS here, that is kept in the terminal extension as it can only be used with a running instance of JupyterLite and here we want to be able to test the commands just using the basic WASM filesystem. There are also some builtin commands (currently only cd) that access the Emscripten filesystem directly.

The idea here is to merge this and release version 0.0.4 of @jupyterlite/cockle, then jupyterlite/terminal#15 can be rewritten to use this. After that we can do some fast iterations of cockle without having to change terminal, to try to get this into a fully working system.

[jtpio]

Looks like we will probably want to exclude these files from the repo at some point, and generate them automatically during the build process?

[ianthomas23]

Yes, although the details of all that are areas for current investigation. I am hoping that we can have a set of default tools plus some optional ones (e.g. biosciences toolkit) all on emscripten-forge, and modify the jupyter lite build process to do whatever is required for the terminal extension to make use of them. But I don't yet understand if or how that might work.