nix: add latest version of zls to flake.nix

I know not everybody uses ZLS (or LSPs in general) but I think it's very
useful and it's very handy to have it in the `flake.nix` to keep it up
to date with the `zig` version.

As with a lot of my PRs in this project, please consider the following a
disclaimer: I have 0 clue what I'm doing here and if there's a better
way to do what I'm trying to do, let me know!

nix/devshell.nix

@@ -20,6 +20,7 @@
 , wraptest
 , zig
 , zip
+, zls
 , llvmPackages_latest
 
 , bzip2
@@ -72,6 +73,7 @@ in mkShell rec {
     scdoc
     zig
     zip
+    zls
 
     # For web and wasm stuff
     nodejs