lint: Use Prettier for JavaScript files.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

prettier.config.js

@@ -1,3 +1,19 @@
 module.exports = {
     bracketSpacing: false,
+    tabWidth: 4,
+    trailingComma: "all",
+    overrides: [
+        {
+            files: ["frontend_tests/casper_tests/*.js", "frontend_tests/casper_lib/*.js"],
+            options: {
+                trailingComma: "es5",
+            },
+        },
+        {
+            files: ["**.yml", "**.yaml"],
+            options: {
+                tabWidth: 2,
+            },
+        },
+    ],
 };