Make dev mode one command and add VS Code config

npm run dev now runs scripts/dev-env.ts first, which creates .dev/site.env with
a known dev password and a random session secret, so a fresh clone serves a
working admin without anyone hand-building a hash. Adds .vscode launch
configurations and tasks for the dev server, one test file, and check-site, and
brings scripts/ under typecheck, which it had been missing.

package.json

@@ -8,7 +8,9 @@
   },
   "scripts": {
     "start": "node src/server.ts",
+    "predev": "node scripts/dev-env.ts",
     "dev": "SECURE_COOKIES=false node --watch src/server.ts",
+    "dev:env": "node scripts/dev-env.ts",
     "seed": "node scripts/seed-dev.ts",
     "check-site": "node scripts/check-site.ts",
     "verify": "node src/cli/verify.ts",