v2.3: General maintence, add alternative to IFD, allow adjacent directories, make targetless

Hello.

Many changes have been made, but most are internal just to clean up the code-base and should not effect usage. If it does, please open an issue.

There are some user-facing change though:

1. Lots of documentation changes (thanks @rasmus-kirk!)
2. SOURCE_DATE_EPOCH is now set in devShells
   • Means that devShells should be more reproducible now
3. the --root flag is now set when calling typst
   • This allows for documents to depend on files in adjacent directories, when previously it would fail compilation.
   • See this test for what would fail before
   • Thanks @songpola!
4. Restructuring of the repo (not user-facing)
5. Remove the assertion about format types
   • This means that when Typst adds new formats in the future, I don't have to add new ones to the assertion. Typst will just fail to compile so there's no real reason to have this assertion
6. Add alternative to IFD
   • Previously, this library unconditionally did import-from-derivation. I forgot that it did that. So I added an alternative since IFD is bad in CppNix/Lix
   • See details here and/or here
   • In the future I hope to deprecate the ad-hoc packages here, and encourage using Nixpkg's pkgs.buildTypstPackage. But some more work is needed in Nixpkgs before that.
   • Now you can specify your extra packages with a specification like

     {
       pname = "cool-package";
       version = "1.0";
       src = coolSrc;
       meta.license = lib.licenses.mit;
     }

7. Improve type assertion for extraPackages
8. Fix overrides for pname and version
9. Make targetless
   • GCC made things too complicated for the rest of the sane compilers
   • Target bad
   • Typst is a targetless compiler because it's not insane, and also it just makes PDFs for the most part
   • Now you can build PDFs for any platform you can dream of, hooray