feat(workflows): auto-assign PR author (#1388)

Motivation
----------
I've heard today that we want to assign each PR to its respective
author, so the name appears in the PR list on the right side:
https://github.com/dreammall-earth/dreammall.earth/pulls

I keep forgetting this, so I'd say let's drop the requirement or
automate this.

Feel free to reject this PR in case we drop the requirement.

How to test
-----------
1. Merge this into `master`
2. See if new PRs get auto-assigned

.github/auto_assign.yml

@@ -0,0 +1 @@
+addAssignees: author

.github/workflows/pr.yml

@@ -12,6 +12,13 @@ permissions:
   statuses: write
 
 jobs:
+  assign-author:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: kentaro-m/[email protected]
+        with:
+          configuration-path: '.github/auto-assign.yml'
+
   main:
     name: Validate PR title
     runs-on: ubuntu-latest