CI: add workflow.

.github/FUNDING.yml

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+patreon: whitequark

.github/workflows/ci.yml

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+on: [push, pull_request]
+name: CI
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out source code
+        uses: actions/checkout@v3
+      - name: Set up Node
+        uses: actions/setup-node@v3
+        with:
+          node-version: v20.x
+      - name: Install dependencies
+        run: npm install
+      - name: Typecheck code
+        run: npm run tsc
+      - name: Lint code
+        run: npm run lint
+      - name: Package extension
+        run: npm exec vsce package
+      - name: Upload package artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: package
+          path: '*.vsix'