Add devbox config

[seven1m]

I've been using Devbox for some other projects and I wanted to see if using it here could simplify the setup process for new devs.

TODO: there are few failing specs (when run within a devbox shell) I need to figure out.

• ::crypt() gives EINVAL
• spec/core/file/flock_spec.rb segfault
• spec/core/io/write_spec.rb Errno::ENOENT

[seven1m]

I'm closing this agian.

There were two issues:

1. Clang compiled for Nix is just too slow. From my limited understanding, it's due to reproducibility -- certain optimizations are disabled. On my machine, the command clang++ -pthread -g -Wall -Wextra -Werror -Wunused-result -Wno-missing-field-initializers -Wno-unused-parameter -Wno-unused-variable -Wno-unused-but-set-variable -Wno-unused -value -Wno-unknown-warning-option -Wno-trigraphs -Wno-vla-cxx-extension -DNAT_GC_GUARD -fPIC -I /home/tim/pp/natalie/include -I /home/tim/pp/natalie/ext/tm/include -I /home/tim/pp/natalie/ext/minicoro -I /home/ tim/pp/natalie/build -I /home/tim/pp/natalie/build/onigmo/include -I /home/tim/pp/natalie/build/prism/include -std=c++17 -DNAT_OBJECT_FILE -shared -fPIC -rdynamic -Wl,-undefined,dynamic_lookup -o build/libnat.s o build/libnat.rb.cpp build/libnatalie.a (part of our build) goes from 32 seconds to 2:53. The test suite takes 4-5x longer to run. It's painful. GCC is even worse... I didn't even measure it.
2. There is a bug in our GC exposed by these versions of Clang and GCC. I'll file that separately... [edit: GC bug: Args values are getting collected prematurely #2345]

There is some good work in this PR that I plan to ship as separate PRs, just without the devbox config.