feat: enable ESM support via dual-publishing

- Configure tsdown to build both CJS and ESM outputs.
- Update package.json exports to support both 'require' and 'import'.
- Fix type hygiene issues by using explicit 'export type'.
- Update protos/update.sh to generate ESM version of compiledFirestore and fix its imports for Node.js compatibility.
- Configure build aliases to handle relative paths to protos correctly in both CJS and ESM builds.
- Add packaging integration test to CI.

Author: taeold

.github/workflows/test.yaml

@@ -46,6 +46,27 @@ jobs:
           key: ${{ runner.os }}-node-${{ matrix.node-version }}-${{ hashFiles('**/package-lock.json') }}
       - run: npm ci
       - run: npm run test
+  packaging:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version:
+          - 18.x
+          - 20.x
+          - 22.x
+          - 24.x
+    steps:
+      - uses: actions/checkout@v1
+      - uses: actions/setup-node@v1
+        with:
+          node-version: ${{ matrix.node-version }}
+      - name: Cache npm
+        uses: actions/cache@v4
+        with:
+          path: ~/.npm
+          key: ${{ runner.os }}-node-${{ matrix.node-version }}-${{ hashFiles('**/package-lock.json') }}
+      - run: npm ci
+      - run: npm run test:packaging
   integration:
     needs: "unit"
     runs-on: ubuntu-latest