chore: typecheck and run tests in prepublish

Author: Loïc Guychard

package.json

@@ -72,7 +72,7 @@
     "build": "parcel build --no-minify --out-file sourcegraph-git-extras.js src/extension.ts",
     "symlink-package": "mkdirp dist && lnfs ./package.json ./dist/package.json",
     "serve": "npm run symlink-package && parcel serve --no-hmr --out-file sourcegraph-git-extras.js src/extension.ts",
-    "sourcegraph:prepublish": "yarn run build"
+    "sourcegraph:prepublish": "yarn typecheck && yarn test && yarn build"
   },
   "commitlint": {
     "extends": [