Add post install.

metacall.rb

@@ -159,6 +159,27 @@ def install
     system "cmake", "--build", ".", "--target", "clean"
   end
 
+  # NPM Post Install
+  def post_install
+    node_modules = HOMEBREW_PREFIX/"lib/node_modules"
+    node_modules.mkpath
+    # Kill npm but preserve all other modules across node updates/upgrades.
+    rm_r node_modules/"npm" if (node_modules/"npm").exist?
+
+    cp_r libexec/"lib/node_modules/npm", node_modules
+    # This symlink doesn't hop into homebrew_prefix/bin automatically so
+    # we make our own. This is a small consequence of our
+    # bottle-npm-and-retain-a-private-copy-in-libexec setup
+    # All other installs **do** symlink to homebrew_prefix/bin correctly.
+    # We ln rather than cp this because doing so mimics npm's normal install.
+    ln_sf node_modules/"npm/bin/npm-cli.js", bin/"npm"
+    ln_sf node_modules/"npm/bin/npx-cli.js", bin/"npx"
+    ln_sf bin/"npm", HOMEBREW_PREFIX/"bin/npm"
+    ln_sf bin/"npx", HOMEBREW_PREFIX/"bin/npx"
+
+    (node_modules/"npm/npmrc").atomic_write("prefix = #{HOMEBREW_PREFIX}\n")
+  end
+
   test do
     (testpath/"test.js").write <<~EOS
       console.log("Hello from NodeJS")