Load WASM/JS modules dynamically using importScripts

[ianthomas23]

Up until now we have been importing the JavaScript-with-embedded-WebAssembly modules statically at build time just like any other TypeScript/JavaScript code. This PR changes this to dynamically import such code on demand using the WebWorker importScripts function. There are two benefits:

1. It will support (with a little more work) separate JS and WASM files. This will be in a follow-up PR.
2. It will be possible (with a larger amount of more work) to register the WASM commands available at runtime, probably when constructing a Shell object, to support arbitrary terminal functionality written by ourselves or any user.

To dynamically import such modules they have to be available at some URL. At this stage I have implemented a really simple solution to this in the test suite and cockle-playground (Jupyter terminal extension to follow) of copying the JS/WASM files from the cockle npm package into the webpack/rspack statically-served assets directory.

The imported JS/WASM modules are cached to make subsequent command calls faster. The first time coreutils is used it takes on my dev machine ~150 ms to import compared to the ~60 ms previously. But subsequent calls only take ~15 ms compared to the ~30 ms previously. grep is faster as it is a much smaller module with just a single command, taking ~60 ms to load the first time and <10 ms subsequently. If these times get worse and feel like they are too long then we can always pre-emptively warm up the cache at the start. Also note that the browser caches the JS/WASM files and if you have a copy of coreutils already cached in the browser it only takes ~60 ms to import the first time.