Group dependabot dependencies

It seems you can group the PRs you get for dependabot version bumps. Let's do this to reduce the number of PRs opened.

Closes #1144.

Author: alexdewar

.github/dependabot.yml

@@ -9,7 +9,13 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    groups:
+      rust:
+        patterns: ["*"]
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
       interval: "weekly"
+    groups:
+      github-actions:
+        patterns: ["*"]