feat: add justfile

[nuke-web3]

Mainly to build the workspace enforcing SP1 ELF is present, as if it's not, cargo build fails.

A followup PR will add a README section on how to install and use just:

# one way to install just
cargo install just

# with just installed run:
just
# ^^ lists the commands

just b
# ^^ runs just build & a _pre-build recipe that checks for a required SP1 ELF file to build the project & generates iff needed

Opens the door to better devex in setting up, building, and deploying things from this repo.