Add CI step to ensure lockfile v2

.drone.yml

@@ -4,8 +4,15 @@ type: docker
 name: node16
 
 steps:
+- name: check-lockfile
+  image: node:16
+  commands:
+  - node ./check-lockfile.js
+
 - name: install-deps
   image: node:16
+  depends_on:
+  - check-lockfile
   commands:
   - npm install
 

check-lockfile.js

@@ -0,0 +1,10 @@
+const lockfile = require('./package-lock.json')
+
+if (lockfile.lockfileVersion !== 2) {
+  console.error(`
+Please upgrade to npm v7 and revert changes to the lockfile.
+
+- \`npm i -g npm\` to upgrade.
+`.trim())
+  process.exit(1)
+}