feat(deploy): Venia can be built and deployed separately (magento#498)

- Change all Venia devDependencies to dependencies, so it can be build
from node_modules with only "production deps"
- Add a "venia" CLI to the package which builds and launches a server

This change enables a downstream user to create a package which launches
Venia as simple as this:

```sh
$ npm install -g @magento/venia-concept
$ NODE_ENV=production venia build
$ NODE_ENV=production MAGENTO_BACKEND_URL=https://magento.internal venia start
```

This enables a deployment that does not require cloning, hoisting, and
building the entire pwa-studio repository.

packages/venia-concept/bin/venia.js

@@ -0,0 +1,23 @@
+#!/usr/bin/env node
+
+const execa = require('execa');
+
+const cwd = require('path').resolve('../', __dirname);
+
+function npmRun(cmd) {
+    execa.shell(`npm run -s ${cmd}`, {
+        cwd,
+        stdio: 'inherit',
+        buffer: false
+    });
+}
+
+const commands = ['build', 'start'];
+const command = process.argv.pop();
+if (!commands.includes(command)) {
+    console.error(
+        `Unrecognized command or flag '${command}'. Run 'venia start' to start, or 'venia build' to build.`
+    );
+}
+
+npmRun(command);