Make server → wasm dependency optional

[kpreid]

It should be possible to build the workspace without necessarily running the wasm build. Use cases:

• Build without requiring npm and wasm-pack tools.
  • People may wish to build and play or contribute to all-is-cubes without caring about the web target.
  • I haven't figured out how to get npm to work on Windows on a GitHub Actions runner; right now the CI steps have a special case for this.
• Publish the all-is-cubes-server library without embedding a wasm binary; have it optionally supplied at run-time, or left out entirely, instead. This way we don't have large rarely-useful build artifacts in what we publish to crates.io.

[kpreid]

Status update:

• Webpack is no longer used at all.
• The server can be built without the -wasm dependency.

However, xtask still builds -wasm for all whole-project operations. I am still thinking about how to make this unnecessary.

One possibility is that by splitting into multiple workspaces along some line, all-is-cubes-server could invoke the -wasm build from inside its own build script, thus ensuring that it's available just in time only if needed. The two possible splits are -wasm in its own workspace or -server in its own workspace. Of these two, -wasm is clearly preferable because it also solves problems with target-specific dependencies not compiling elsewhere, but the last time I tried that I got a mysterious run time memory corruption error. So, I want to solve that to be able to solve this properly. #410 tracks that.