chore: add LSIF indexing to CI (#676)

Co-authored-by: Sourcegraph Bot <campaigns@sourcegraph.com>

Author: gbrik

.github/workflows/lsif.yml

@@ -0,0 +1,15 @@
+name: LSIF
+on:
+  - push
+jobs:
+  lsif-ts:
+    runs-on: ubuntu-latest
+    container: sourcegraph/lsif-node
+    steps:
+      - uses: actions/checkout@v1
+      - name: Install dependencies
+        run: npm install
+      - name: Generate LSIF data
+        run: lsif-tsc -p .
+      - name: Upload LSIF data
+        run: src lsif upload -github-token=${{ secrets.GITHUB_TOKEN }}