Add dependabot configuration file

**Context**: I know some updates are needed to the GHA workflows due to new major releases of action runners. May as well set up  Dependabot now to get those updates in (which automatically tests the config).

----

This automatically enables Dependabot to:
* Submit pull requests for security updates and version updates for Composer dependencies.
* Submit pull requests for security updates and version updates for GH Action runner dependencies.

For Composer dependencies, a preference is given to _widen_ the version restrictions instead of updating them to a new minimum.
This is a deliberate choice as this package is a library, not an application.

The configuration has been set up to:
* Run once a week.
* Submit a maximum of 5 pull requests at a time.
    If additional pull requests are needed, these will subsequently be submitted the next time Dependabot runs after one or more of the open pull requests have been merged.
* The commit messages for PRs submitted by Dependabot will be prefixed according the unofficial conventions used in this repo up to now.
* The PRs will automatically be labelled with an appropriate label as already in use in this repo.

Refs:
* https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
* https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#versioning-strategy

Author: jrfnl

.github/dependabot.yml

@@ -0,0 +1,28 @@
+# Dependabot configuration.
+#
+# Please see the documentation for all configuration options:
+# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+
+version: 2
+updates:
+  - package-ecosystem: "composer"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5 # Set to 0 to (temporarily) disable.
+    versioning-strategy: widen
+    commit-message:
+      prefix: "Composer:"
+      include: "scope"
+    labels:
+      - "Type: chores/QA"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5
+    commit-message:
+      prefix: "GH Actions:"
+    labels:
+      - "Type: chores/QA"