Build/Test Tools: Monitor default themes for dependency updates.

There are currently 3 default themes with build processes: Twenty Twenty-One, Twenty Twenty, and Twenty Nineteen. This adds a configuration for each to the Dependabot configuration to ensure pull requests are opened when updates become available.

Because they have a low number of dependencies, they will all be updated in the same pull request to start. Groups can be configured later if this presents problems.

See #63170, #63170.

git-svn-id: https://develop.svn.wordpress.org/trunk@60347 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

.github/dependabot.yml

@@ -127,3 +127,34 @@ updates:
           - "lodash"
           - "moment"
           - "underscore"
+
+  # Monitor npm dependencies within default themes.
+  - package-ecosystem: "npm"
+    directory: "/src/wp-content/themes/twentytwentyone"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 20
+    groups:
+      twentytwentyone:
+        patterns:
+          - "*"
+
+  - package-ecosystem: "npm"
+    directory: "/src/wp-content/themes/twentytwenty"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 20
+    groups:
+      twentytwenty:
+        patterns:
+          - "*"
+
+  - package-ecosystem: "npm"
+    directory: "/src/wp-content/themes/twentynineteen"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 20
+    groups:
+      twentynineteen:
+        patterns:
+          - "*"