[FEATURE] Configure Dependabot to also update dev dependencies (#1295)

If we want to have fixed versions of our development dependencies,
manually updating them will be quite a hassle. Let's automate this
instead using Dependabot.

.github/dependabot.yml

@@ -6,3 +6,11 @@ updates:
     directory: "/"
     schedule:
       interval: "daily"
+
+  - package-ecosystem: "composer"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    allow:
+      - dependency-type: "development"
+    versioning-strategy: "increase"