ci: add initial testing workflow (#25)

tests in non-core repositories aren't entirely working, but this
at least backstops what is currently working

.github/workflows/test.yaml

@@ -0,0 +1,50 @@
+name: test
+on:
+  push:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Install Node
+        uses: actions/setup-node@v3
+        with:
+          node-version: 18
+
+      # https://github.com/pnpm/action-setup
+      - name: Install pnpm
+        uses: pnpm/action-setup@v2
+        with:
+          version: 8.x.x
+
+      - name: Get pnpm store directory
+        id: pnpm-cache
+        shell: bash
+        run: |
+          echo "STORE_PATH=$(pnpm store path)" >> $GITHUB_OUTPUT
+
+      - uses: actions/cache@v3
+        name: Setup pnpm cache
+        with:
+          path: ${{ steps.pnpm-cache.outputs.STORE_PATH }}
+          key: ${{ runner.os }}-pnpm-store-v1-${{ hashFiles('**/pnpm-lock.yaml') }}
+          restore-keys: |
+            ${{ runner.os }}-pnpm-store-v1-
+
+      - name: Install dependencies
+        run: pnpm install
+
+      - name: Build core
+        working-directory: packages/core
+        run: pnpm build
+
+      - name: Test core
+        working-directory: packages/core
+        run: pnpm test
+
+      - name: Build viewer
+        working-directory: packages/viewer
+        run: pnpm build