examples/vite.config.ts: check for required environment variables (#4578)

## Motivation

Make it harder to accidentally deploy the demo apps without setting
their required environment variables.

## Proposal

Add a Vite plugin that checks the requisite variables are set (though
not sensible) at build time.

## Test Plan

Tested manually.

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

## Links

- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

Author: Twey

examples/vite.config.ts

@@ -1,10 +1,26 @@
 import { defineConfig } from 'vite';
 import path from 'path';
 
+const checkEnvironment = envs => {
+  const missing = envs.filter(env => !(env in process.env));
+  if (missing.length > 0)
+    throw new Error(`required environment variables missing: ${missing.toString()}`);
+
+  return {
+    name: 'checkEnvironment',
+  };
+}
+
 // https://vitejs.dev/config/
 export default defineConfig({
   base: '',
   envPrefix: 'LINERA_',
+  plugins: [
+    checkEnvironment([
+      'LINERA_FAUCET_URL',
+      'LINERA_APPLICATION_ID',
+    ]),
+  ],
   server: {
     headers: {
       'Cross-Origin-Embedder-Policy': 'require-corp',