feat: add justfile, setup CI

[chrisdickinson]

• Move the path detection to its own module, utils, so our command runner can test to see if we need to install extism and the shared libraries separately
• I'm experimenting with the just command runner stuff.
  • The justfile prepare recipe is intended for humans (vs. CI) – this is to ensure contributors have the necessary prerequisites. In CI I've installed the dependencies in separate, cacheable steps.
• The poetry deps are cached using actions/setup-python.
• If you know of a better way to determine the download than the munging I'm doing in bash, please let me know! gcc -dumpmachine was too specific, arch doesn't quite return what we want, and uname -a didn't feel great either.