Use GitHub Actions to automatically label Pull Requests (#2420)

* Add workflow file to run PR labeler and corresponding configuration file

I'm using my personal fork of https://github.com/actions/labeler because 
I could not any labeling action that checked whether a file was added or 
changed (or removed).

* Fix typo in Labeler action configuration file

.github/labeler.yml

@@ -0,0 +1,11 @@
+# Configuration file for the Pull Request Labeler bot
+
+# Add `new icon` label when there is an SVG added to the icons/ directory
+"new icon":
+  - on: added
+  - icons/*.svg
+
+# Add `icon outdated` label when there is an SVG modified to the icons/ directory
+"icon outdated":
+  - on: modified
+  - icons/*.svg

.github/workflows/pull_requests.yml

@@ -0,0 +1,11 @@
+name: Pull Requests
+on: [pull_request]
+
+jobs:
+  label:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: ericcornelissen/labeler@master
+      with:
+        repo-token: "${{ secrets.GITHUB_TOKEN }}"