Also run on pull requests

Aligns with the config of fake-timers:
https://github.com/sinonjs/fake-timers/blob/master/.github/workflows/main.yml

Author: fatso83

.github/workflows/main.yml

@@ -1,6 +1,12 @@
 name: Node CI
 
-on: [push]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - '**'
 
 jobs:
   prettier: