Create development shells

Only with the basics for now. I've yet to explore more complex use cases
such as the following repos:

- https://github.com/GTrunSec/Coding-Dev-Env-With-NixFlake
- https://github.com/LavaDesu/flakes

This seems nifty feature especially when doing some project spelunking
or something.

README.adoc

@@ -37,6 +37,7 @@ nixos-config
 ├── modules/
 ├── pkgs/
 ├── secrets/
+├── shells/
 ├── users/
 ├── flake.lock
 ├── flake.nix
@@ -55,6 +56,10 @@ It is exported in the flakes at `outputs.packages` compiled through various syst
 
 * link:./secrets/[`./secrets/`] contains my secrets managed with link:https://github.com/ryantm/agenix[agenix].
 
+* link:./shells/[`./shells/`] contains my development shells for interacting with the usual type of projects.
+Setting this up can bring benefits outside of NixOS (unless you're interacting with projects with any OpenGL-related stuff).
+footnote:[Since packages brought from Nix shells can only work with the store, a container might be better at some situations.]
+
 * link:./users/[`./users/`] contains my link:https://github.com/nix-community/home-manager[home-manager] configurations and modules.
 It is exported in the flakes at `outputs.homeConfigurations`.
 For more information, see the link:./users/README.adoc[related documentation].
@@ -68,7 +73,7 @@ In order of priority:
 
 * [x] Create custom modules.
 * [x] Create a themes system similar to link:https://github.com/hlissner/dotfiles[this NixOS config].
-* [ ] Create development shells.
+* [x] Create development shells.
 * [x] Manage secrets with agenix.
 * [x] Automate backups with NixOS config.
 * [x] Create custom packages and export it to flakes. (Maybe consider making it to upstream)

flake.nix

@@ -121,5 +121,11 @@
       # I just want to try out supporting other systems.
       packages = forAllSystems
         (system: import ./pkgs { pkgs = import nixpkgs { inherit system; }; });
+
+      # My several development shells for usual type of projects. This is much
+      # more preferable than installing all of the packages at the system
+      # configuration (or even home environment).
+      devShells = forAllSystems
+        (system: import ./shells { pkgs = import nixpkgs { inherit system; }; });
     };
 }

shells/default.nix

@@ -0,0 +1,5 @@
+{ pkgs ? import <nixpkgs> { } }:
+
+with pkgs; {
+  flatpak = callPackage ./flatpak.nix { };
+}

shells/flatpak.nix

@@ -0,0 +1,12 @@
+# My custom shell for developing Flatpak manifests.
+{ mkShell, lib, flatpak-builder, editorconfig-checker, editorconfig-core-c, git, dasel }:
+
+mkShell {
+  packages = [
+    dasel # For converting various data into something.
+    flatpak-builder # A required tool.
+    editorconfig-checker # We're most likely writing manifests in YAML so I need them consistent spaces.
+    editorconfig-core-c # editorconfig will not work without the engine, of course.
+    git # This is the common choice as the VCS — otherwise, bring your own.
+  ];
+}