chore(nix): add flake for developing and building lpm

[Silzinc]

I managed to build lpm with this. Now, there are a few clear issues:

• The resulting lpm binary is injected into the devshell (I had the impression that it was necessary to dev lpm, am I correct?). If this commit is merged, I will be able to grab the flake from this github repo and have a lpm build at a fixed revision.
• I have to go through the endeavour of fetching tarballs from lj-dist on the nix side and point luajit towards its location, because the access to internet is restricted by default in the build phase on the bash/lua side and I do not assume access to /tmp on a nix system. I will probably have to make a derivation for lj-dist and build libluajit directly instead of fetching it, though it will probably require a few changes on the Lua side.

[codebycruz]

Assuming this pull request isn't a WIP anymore despite the name. In the future I'll probably mark these as draft to use GitHub's functionality which might be more clear.

Just some nitpicks pretty much, I think the flake can be simplified to be smaller but it is fine as is and I don't want to redo your gen-release script, although it would be nice to simplify that since this PR will take the repo to <99% Lua usage :(

[codebycruz]

Can the changes in this file be undone now?

[codebycruz]

Can probably hoist the v0.7.1 into a separate variable to deduplicate code and make it easier to update

[codebycruz]

At the moment stylua isn't necessary as LuaLS' formatter is being used. Additionally, luajit is not necessary unless you're using BOOTSTRAP=1 which is no longer used in the flake

[Silzinc]

It'll certainly be back above 99% soon enough :)