feat: add nix flake

(cherry picked from commit 71e9e42)

.envrc

@@ -0,0 +1 @@
+use flake

.gitignore

@@ -1,6 +1,7 @@
 # Miscellaneous
 *.class
 *.lock
+!flake.lock
 *.log
 *.pyc
 *.swp
@@ -96,3 +97,5 @@ app.*.map.json
 
 # VS Code
 .vscode/settings.json
+
+.direnv

flake.nix

@@ -0,0 +1,50 @@
+{
+  description = "Vikunja Flutter app dev environment";
+
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let pkgs = import nixpkgs { system = system; config.allowUnfree = true; };
+      in {
+        defaultPackage = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            # Originally copied from https://github.com/zulip/zulip-flutter/blob/d882f50bf7d917ce7e040d7ee1f66f1d803ef988/shell.nix
+              clang
+              cmake
+              ninja
+              pkg-config
+
+              gtk3  # Curiously `nix-env -i` can't handle this one adequately.
+                    # But `nix-shell` on this shell.nix does fine.
+              pcre
+              epoxy
+
+              flutter
+
+              # This group all seem not strictly necessary -- commands like
+              # `flutter run -d linux` seem to *work* fine without them, but
+              # the build does print messages about missing packages, like:
+              #   Package mount was not found in the pkg-config search path.
+              #   Perhaps you should add the directory containing `mount.pc'
+              #   to the PKG_CONFIG_PATH environment variable
+              # To add to this list on NixOS upgrades, the Nix package
+              # `nix-index` is handy: then `nix-locate mount.pc`.
+              libuuid  # for mount.pc
+              xorg.libXdmcp.dev
+              libsepol.dev
+              libthai.dev
+              libdatrie.dev
+              libxkbcommon.dev
+              dbus.dev
+              at-spi2-core.dev
+              xorg.libXtst.out
+              pcre2.dev
+
+              jdk11
+              android-studio
+              android-tools
+          ];
+        };
+      });
+}