dev: add commit linter workflow

Experimenting with an internal linter. Only trouble is, if Doom's CLI is
ever in a broken state, a contributor will have to know about
--no-verify option for git commit. This can be improved post CLI
rewrite.

.github/workflows/lint.yml

@@ -0,0 +1,11 @@
+name: Commit linter
+on:
+  pull_request:
+jobs:
+  lint-commits:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2.3.1
+        with:
+          fetch-depth: 0
+      - run: bin/doom ci lint-commits HEAD~1