build: Use prettier for javascript

Stop ignoring .js and .json files in prettier and fix existing issues.

The `prettier-plugin-multiline-arrays` plugin is used to split arrays
with multiple elements into one line per element.

proxy.conf.js

@@ -32,7 +32,9 @@ function setupForCorporateProxy(proxyConfig) {
   if (proxyServer) {
     console.log(`Using corporate proxy server: ${proxyServer}`);
     agent = new HttpsProxyAgent(proxyServer);
-    proxyConfig.forEach(entry => { entry.agent = agent; });
+    proxyConfig.forEach((entry) => {
+      entry.agent = agent;
+    });
   }
 
   return proxyConfig;