flake: add devShells

exposed via nix registry :D
can be used in nix-direnv like `use flake shells#cs.osu`

shells/flutter/default.nix

@@ -0,0 +1,53 @@
+{
+  lib,
+  mkShell,
+
+  flutter,
+  clang,
+  cmake,
+  ninja,
+  pkg-config,
+
+  atk,
+  cairo,
+  gdk-pixbuf,
+  glib,
+  gtk3,
+  harfbuzz,
+  libGL,
+  pango,
+  wayland,
+  xorg,
+}:
+let
+  makeIncludePath = lib.makeSearchPathOutput "include" "include";
+
+  includePath = makeIncludePath [
+    xorg.xorgproto
+    xorg.libX11.dev
+  ];
+in mkShell {
+  nativeBuildInputs = [
+    flutter
+    clang
+    cmake
+    ninja
+    pkg-config
+  ];
+
+  C_INCLUDE_PATH = includePath;
+  CPLUS_INCLUDE_PATH = includePath;
+
+  LD_LIBRARY_PATH = lib.makeLibraryPath "/lib" [
+    atk
+    cairo
+    gdk-pixbuf
+    glib
+    gtk3
+    harfbuzz
+    libGL
+    pango
+    wayland
+    xorg.libX11
+  ];
+}