chore(ci): add upmerge job

.github/workflows/upmerge.yml

@@ -0,0 +1,19 @@
+name: 'Upmerge main to next'
+
+on:
+  push:
+
+jobs:
+  upmerge:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+
+    - name: Upmerge
+      uses: bambamboole/gha-upmerge@master
+      with:
+        stable_branch: 'main'
+        development_branch: 'next'
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}