Add CI workflow

Builds and runs tests on Postgres 14 - 16. Couldn't get it to
automatically init the submodule, so doing it manually. One test fails
on my system; let's see if it works on GitHub.

Author: theory

.github/workflows/ci.yml

@@ -0,0 +1,20 @@
+name: CI
+on:
+  push:
+    branches: ["*"]
+  pull_request:
+  schedule:
+    - cron:  '0 12 15 * *' # Monthly at noon on the 15th
+jobs:
+  build:
+    strategy:
+      matrix:
+        pg: [16, 15, 14]
+    name: 🐘 PostgreSQL ${{ matrix.pg }}
+    runs-on: ubuntu-latest
+    container: pgxn/pgxn-tools
+    steps:
+      - run: pg-start ${{ matrix.pg }}
+      - uses: actions/checkout@v4
+      - run: git submodule update --init --recursive
+      - run: pg-build-test

.gitignore

@@ -1,5 +1,6 @@
 *.o
 *.so
+*.dylib
 results
 regression.diffs
 regression.out