chore: add publish to pub.dev

.github/workflows/publish.yaml

@@ -0,0 +1,16 @@
+name: Publish to pub.dev
+
+on:
+  push:
+    tags:
+      - "v[0-9]+.[0-9]+.[0-9]+" # tag-pattern on pub.dev: 'v{{version}}'
+
+# Publish using the reusable workflow from dart-lang.
+jobs:
+  publish:
+    permissions:
+      id-token: write # Required for authentication using OIDC
+    uses: dart-lang/setup-dart/.github/workflows/publish.yml@v1
+    with:
+      environment: pub.dev
+      working-directory: packages/disco