Replace spidermonkey-wasi-embedding submodule with gecko-dev

[JakeChampion]

This change makes it so that contributions which require modifying our branch of gecko-dev, don't require having to edit 3 repositories, and instead just 2 (gecko-dev and this repo).

We do currently lose prebuilt spidermonkey-wasi builds, which does mean a contributor will need to clone and compile gecko-dev. I've added a script which will install the required dependencies for gecko-dev, named install-gecko-dependencies.sh, and also updated our Makefile to compile gecko-dev.

make will compile gecko-dev and js-compute-runtime, both in release mode
DEBUG=true make will compile gecko-dev and js-compute-runtime, both in debug mode

[elliottt]

What do you think about keeping the runtime/spidermonkey directory so that the build happens in a completely different tree? I think we should still try to make it easy to keep the two codebases separate so that we stick to the embedder api.