Refactor template: extract scripts, overlays, and hosts into separate modules

Co-authored-by: dwinkler1 <22460147+dwinkler1@users.noreply.github.com>

templates/rde/overlays/r.nix

@@ -0,0 +1,20 @@
+# R packages overlay
+final: prev: let
+  reqPkgs = with final.rpkgs.rPackages;
+    [
+      broom
+      data_table
+      janitor
+      languageserver
+      reprex
+      styler
+      tidyverse
+    ]
+    ++ (with final.extraRPackages; [
+      httpgd
+    ])
+    ++ (prev.lib.optional (builtins.pathExists ./r-packages.nix) (import ./r-packages.nix final.rpkgs));
+in {
+  quarto = final.rpkgs.quarto.override {extraRPackages = reqPkgs;};
+  rWrapper = final.rpkgs.rWrapper.override {packages = reqPkgs;};
+}