[6.x] Preinstall check that mentions 'yarn kbn' (#16572)

package.json

@@ -41,6 +41,7 @@
     "Yuri Astrakhan <yuri@elastic.co>"
   ],
   "scripts": {
+    "preinstall": "node ./preinstall_check",
     "kbn": "node scripts/kbn",
     "test": "grunt test",
     "test:dev": "grunt test:dev",

preinstall_check.js

@@ -0,0 +1,32 @@
+const isUsingNpm = process.env.npm_config_git !== undefined;
+
+if (isUsingNpm) {
+  throw "Use Yarn instead of npm, see Kibana's contributing guidelines";
+}
+
+// The value of the `npm_config_argv` env for each command:
+//
+// - `npm install`: '{"remain":[],"cooked":["install"],"original":[]}'
+// - `yarn`: '{"remain":[],"cooked":["install"],"original":[]}'
+// - `yarn kbn bootstrap`: '{"remain":[],"cooked":["run","kbn"],"original":["kbn","bootstrap"]}'
+const rawArgv = process.env.npm_config_argv;
+
+if (rawArgv === undefined) {
+  return;
+}
+
+try {
+  const argv = JSON.parse(rawArgv);
+
+  if (argv.cooked.includes('kbn')) {
+    // all good, trying to install deps using `kbn`
+    return;
+  }
+
+  if (argv.cooked.includes('install')) {
+    console.log('\nWARNING: When installing dependencies, prefer `yarn kbn bootstrap`\n');
+  }
+} catch (e) {
+  // if it fails we do nothing, as this is just intended to be a helpful message
+}
+