Add pgxn-releas workflow

Also add .gitattributes so that `git archive` excludes files not needed
in the release archive.

.gitattributes

@@ -0,0 +1,3 @@
+.gitignore export-ignore
+.gitattributes export-ignore
+.github export-ignore

.github/workflows/pgxn-release.yml

@@ -0,0 +1,20 @@
+name: ⚙️ Release on PGXN
+on:
+  push:
+    # Release on semantic version tag.
+    # tags: ['v[0-9]+.[0-9]+.[0-9]+']
+jobs:
+  release:
+    name: Release on PGXN
+    runs-on: ubuntu-latest
+    container: pgxn/pgxn-tools
+    steps:
+    - name: Check out the repo
+      uses: actions/checkout@v4
+    - name: Bundle the Release
+      run: pgxn-bundle
+    - name: Release on PGXN
+      env:
+        PGXN_USERNAME: ${{ secrets.PGXN_USERNAME }}
+        PGXN_PASSWORD: ${{ secrets.PGXN_PASSWORD }}
+      run: pgxn-release