Add IDX template configuration to Gemini samples (#2344)

Adds the configuration needed to open a subdirectory as a template in
IDX. Configuration is added to two samples that use Gemini -- Dynamic
theme and Gemini tasks

gemini_tasks/idx-template.nix

@@ -0,0 +1,15 @@
+# No user-configurable parameters
+{ pkgs, ... }: {
+  packages = [
+     pkgs.flutter
+  ];
+  # Shell script that produces the final environment
+  bootstrap = ''
+    export HOME=/home/user
+    export PATH="$PATH":"$HOME/flutter/bin"
+
+    cp -rf ${./.} "$out"
+    chmod -R +w "$out"
+    rm -rf "$out/.git" "$out/idx-template".{nix,json}
+  '';
+}