add: unit and integration testing

app/node_modules/update-browserslist-db/check-npm-version.js

@@ -0,0 +1,20 @@
+let { execSync } = require('child_process')
+let { existsSync } = require('fs')
+let pico = require('picocolors')
+
+if (!existsSync('deno.lock')) {
+  try {
+    let output = execSync('npm -v').toString().trim()
+    let version = parseInt(output.replace(/^[^\d]*/, ''))
+    if (version <= 6) {
+      process.stderr.write(
+        pico.red(
+          'Update npm or call ' +
+            pico.yellow('npx browserslist@latest --update-db') +
+            '\n'
+        )
+      )
+      process.exit(1)
+    }
+  } catch {}
+}