Add nix flake with devshell, formatting, build and checks

[bitfl0wer]

Description

this nix flake is heavily based off of prior work i have done in one of my repos. it comes with the following features:

• building sable: nix build .#sable or just nix build. creates a result/ dir with the built application.
• devshell: nix users now no longer need to do download and install all the dependencies for sable by themselves. the devshell comes with all the dependencies needed to develop sable
• checks: nix flake check is configured to lint and test:run the application, in addition to checking for correct formatting
• formatting: nix fmt runs all the formatters pnpm would also call, but provides a more familiar interface for nix users.
• pre-commit checks: nix users can benefit from having their changes checked for before the commit goes through. yay :3

.vscode/settings.json was updated to include hints for VS Code users using the "nix env selector" extension to auto-detect the nix dev environment, if you have nix installed.

I had to change oxfmt.config.ts just a tiny bit to make it work with the nix stuff, because node_modules does not exist inside the nix build sandbox. pnpm fmt successfully runs, so nothing should have broken.

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

[bitfl0wer]

After reading through CONTRIBUTING.md more thoroughly, I decided to add all 5 checks mentioned there into nix flake check, instead of just test:run, lint and fmt:check.

edit(5e2fccc): just changed email so that the commit is now verified :)